As an obvious tip, take it slow and be on your guard all the time. It is essential that you block, counter, and shoot anything that gets close. If you shoot before blocking and countering an alien most times it will explode into a pool of acid and kill you instantly.
When you get involved in firefights with many aliens try and backstep toward a corner shooting and aiming carefully...as you reach the corner sprint through the middle of the attacking group slowly picking them off and backstep toward another corner, rinse and repeat.
Always use turrets to your advantage...lure aliens into its sights to help save ammo...and yourself!
Also keep a shotgun and scope rifle with you. The scope highlights enemies through walls as well as the invisible predator when you fight it in the arena. Don't waste shotgun secondary attack unless it is against tough enemies - like Preatorians and the Predator.
Mission 1: The Colony
Fairly simple mission up to the club. Choose one direction and run around, this part can be done with just the pistol, run, turn, shoot, run again. If it beeps infront of you when you run, just run straight past it, get some distance, turn, shoot. etc. Keep your distance and it shouldnt be a problem.
After you reactivate the turret with Echo team watch the bottom left vent for a xeno, make quick work and run upstairs, kill the next 3 xenos (you can lure them to the active turret if you want). Run up to the barricade and spray with the assault rifle, after about 30-40 seconds the door behind will open. Follow the marker to the elevator door, (its easy at this part to just block and counter any attacking alien) after another 15-20 seconds the elevator will open.
In the sewer proceed slowly round the first corner, spawn the xeno, kill it, then proceed round to kill the next xeno that spawns on another corner, and then again a third time. Right after this there is some rifle ammo on the floor. Pick it up and SPRINT to the end, dont turn to shoot anything, hug the right side of the tunnel to avoid 2/3 aliens on the left wall - i made the mistake of hugging the left side, shooting one, and dying from a pool of acid. Sprint to the other side of the small circular blue pillar thingy to end the level.
Mission 2: The Refinery
Go through to the alien shadow on the wall, as the alien runs off shoot a round or two at it and an alien will come round the right corner. Take your time round each corner, picking off any xeno you come across. Proceed through the objectives into and out the outbuilding, taking it slow and try to only use the pistol, picking up all ammo and weapons you come across - one xeno lurks in the room with the Pulse rifle, and extra shotgun shells can be found next to a hole on the same floor the elevator stops at on your way out.
At the bridge reload the shotgun, and have the pistol out ready. As you cross the bridge 6/7 xenos come. Dont forget to use the explosive tanks when xenos group around them. You'll probably still have 5 get across the bridge after pistolling a couple and using the explosives. Shotgun at the ready - WAIT, for them to attack you, block and counter to knock them to ground and then shoot them with a single shot at close range (RT not LT). If you dont wait to block and attack, most times you will shoot them mid attack and spill acid over you. Try and save shotgun ammo and use the Pulse Rifle if some xenos wont die from 2 shotgun shots.
Continue over the bridge and cautiously through the mine - a face hugger will come out under the large fan. When at the hackable door collect the ammo and quickly run back round the corner to familiarise yourself with the layout as you may have to run backwards shooting the xenos that come at you. When you hack the door the xenos can initially come from either side, once you know where lure them to the canisters and use them, backpeddle round the corners (not into a corner) and unload whatever you like - once again it is possible to just use the pistol but you will need to block and counter effectively. Remember to glance at your beeper as although you can backpeddle away the xenos do use the vents to flank to your side and even behind you.
Proceed onward, following the markers, through the twisty room killing 3 xenos, another xeno in the corridor after that, down the stairs, and into the hive. You can shoot at the eggs and then face huggers at a distance, or if you want, run right up to one, and just as it opens (i really do mean as it ONLY JUST STARTS to open) use pistol secondary fire twice right on the rim as it opens to kill a facehugger quickly - if there are multiple eggs right next to eachother kill them at distance instead. Dont bother with the flamethrower, just pistol Zandt in the face. Half a dozen xenos will come for you. You can pistol the first one or two, then use the pulse rifle or shotgun. Remember the 'block, counter, shoot on the floor' with the shotgun if they are at close range.
Up the elevator shoot the xenos on the walls, the eggs in the green mist in the middle can be left alone - if you dont need the health or drop in accidentaly. There are 6 xenos in your way to the marker and 3 more eggs. In the egg filled room take your time killing all the face huggers with the pistol and let them come to you before flicking the switch. If you want to be daring you can flick the switch asap and then grenade launcher the glowing hot pipes to kill all the eggs at once - though a couple of facehuggers might be around to jump on you and end your mission for you.
When the xenos come they continously respawn so shoot them with the shotgun in the legs to cripple them. If they are crippled and not dead it will prevent more from spawning - they do bleed out though, so pay attention. As soon as the switch is ready again press it to end the mission.
Mission 3: The Jungle
Replace the flamethrower with the scoped rifle. You can one shot kill most enemies, particularly because the long heads make headshots way too easy, (particularly for those that play FPS) and for weaker enemies like the acid spitters sometimes a single shot in the leg or body will also put them down for good. Those with surround sound will be able to hear most enemies first, look in the general direction and use the scope zoom to pinpoint them through walls, in trees, and in dark areas. Follow the marker into the cave, where 2 eggs will open, and a 3rd facehugger is by the hack point. Go through the swamp, press the switch, proceed to the marker while killing the xenos that come for you.
The compound is simple enough, grab a power node from one of the outbuildings - one has a xeno, another a facehugger, and a shotgun in the last one. Don't be caught off guard by a xeno in the main building, upstairs, on the wall just on the left through the door. Sprint outside the main gate watching for the red lasers of the predator.
Continue throught the jungle where there will be several acid spitters on the way, its easy to sidestep the acid, and even easier to fire the scope rifle from the hip and kill everything before they have chance to spit anything. When you see the big doors at the end use the scope rifle - it will highlight one of the androids on the floor, he will get up if you get too close...aim for his legs and put 2 shots in them to put him down before he even gets up. When you open the door, step forward and fire a random shot, retreat to the corner and peek round it. Aim down the scope and shoot - anywhere between 4-6 body shots can kill them but its easier to save ammo by going for the limbs. One of 3 androids has a rifle himself, and easily kills you in 2 shots, and rarely misses. Hop back round the corner if one or two decide to rush you, use the scope to look through the wall, use the shotgun if they get too close.
Mission 4: Ruins
Run forward and replace the pulse rifle with a scope rifle, as you go forward the first beep is a phantom and cant be seen and wont attack you. Listen for the screams of the aliens, use the rifle to find them and put them down. When you hack the door up the stairs 3 xenos will come for you, shotgun them. There are two androids in the next room on the ground floor - use the rifle to find them and take them out. If you run into the middle of the room you will be shot upon from above. Another androis will be up a set of stairs next to an explosive canister, two more are upstairs, one behind a wall on the left through the doorway, and one at the far back infront of you. There is a 3rd inactive on the floor on of the corners. Use the rifle to find the one on the floor.
In the next area grab the smart gun and burst fire xenos at mid-long distance. You can kill the first wave with the first clip. Watch your left as an acid spitter is on the wall and can be overlooked. Follow the marker, watch the beeps, burst the smart gun. When you get to the dropship itself another few xenos come, again, use the smart gun. When its over, run over to where you dropped your weapons for the smart gun and pick them up again (shotgun/scope rifle).
The ruins themselves are easy to get through, stick with the rifle, follow the beeps, shotgun anything that gets too close.
The arena with the predator is fairly simple...use the rifle to follow his invisible cloak and shoot him whenever he is stationary, particularly when you see the red lasers aiming at you. As soon as his shield is down he seems to retreat to the same pillar, shoot whatever you have into him before he regenerates his shield. Eventually when he is down to about 1/4 health he WILL charge you. Get out your old shotgun pal and blast double shot (LT) into him and he'll go down without much fuss.
Getting to Tequila and following her to the end of the level is easy, just rifle everything with headshots.
Mission 5: Research Lab
Scope rifle to see the android on the floor at the start. Follow Tequila, shoot the xeno mauling on a corpse. In the large room two androids will come out at the other side, you will have time to kill one and weaken another before the tram gets in your way. You can play the tram part risky if you want to stand against the back wall looking into the tram as the door opens and emptying your rifle on one or two of the androids. A shotgun android will try flanking to your right if you dont shoot him first, and another will move down to your left next to the pillar. You can hang back over by Tequila if you wish, just wait for them to pop out so you can shoot them. Move into the tram and to the left, take aim and move slowly out, picking off one at a time. When all androids are dead move up and kill the two xenos that spawn.
Continue following Tequila. Up the elevator and through the first doors an android will reactivate on the floor. Use the scope to once again pick out the culprit and put two shots his legs. Continue through the holding area to your marker and put Tequila in stasis.
Once you have spoken to Katya pick up the ammo and head to the door. Aim for the android past the barricade, but dont move too far out or a sentry turret will rip you apart. Follow the marker killing xenos and a couple of androids in a room, another android is on a walkway above the same room, but he sometimes gets killed by the xenos. Out the next door a xeno comes through the floor next to the turret on your right. Dont go round th next corner as there is a sniper android on the walkway, use your scope to find him, edge out of cover and take him out. There are two more androids down the corridor, as you move down it two xenos will come out the floor.
For the next series of rooms and corridors go slowly, hide round corners and use the scope when you hear the beeps. Androids will be waiting to ambush you, and in the first area, when the androids are dead some xenos come along. In the first corridor two androids will reactivate, the front left one has a rifle and can easily kill you if you arent careful. Once again use your own rifle to put rounds in their legs before they get up. Over the walkway there are three androids and one at the end of the next corridor by an elevator. When you hear the elevator activate back up because two androids with shotguns will come out.
When out the elevator activate the switch, in the next room is a facehugger. Go out the building and follow the walkways, xenos will jump onto the walkways in a couple of places, line up the rifle for easy headshots when they land. Move inside and pick up all ammo available, when you get to the large room a Preatorian will jump in, back up to where you first came in, rifle the three normal xenos and get the shotgun ready. Double shot the Preatorian with a shotgun, you will need to reload it before its dead, so stick behind the pillars and use them for cover. Once its dead follow the marker to the big elevator to end the mission.
Mission 6: The Pyramid
The only place i had any real trouble was the last mission because you have to fight through a few dozen aliens...then 8 invisible shotgun weilding androids, and finally Weyland.
At the first part run to the turret and activate it, (you can pick up the smart gun if you want, but i saved it for harder enemies later - the praetorians and invisible androids) If you dont pick up the smart gun get out a scope rifle and sit in the big doorway you came in, aim down the bridge and you can take out at least 10-15 aliens before too many get past the turret. Switch to the Pulse rifle and start running around in circles, hug the corner of the statues in the first area and then run along the bridge so you can use the turret to your advantage again. As they once again start massing together sprint through and back down the bridge killing anything following too close with the Pulse rifle as you go. Rinse and repeat the process.
As soon as there are no more aliens run to where there are two health packs by a statue and aim at the edge where an Alien will come up. As this alien comes up there will now be 2 preatorians standing on the bridge. Now pick up the smart gun (if you havent already). If the preatorians arent coming at you, tap the trigger at the one at the front (if you hit both, they will both run at you, if that happens, you might as well start the mission again as they rip you apart before you get to kill one, or if you do, you'll have a long time to wait for the smart gun to reload). Just make sure that the preatorians come one at a time, use the smart gun in bursts at distance then hold the trigger when they are right infront of the turret.
Before moving over the bridge keep the smart gun and pick up the rest of the smart gun ammo. Should say about 150/250 in the ammo counter. (If you already used it all take the Scope rifle and shotgun for the next part) Continue through to the cutscene with Weyland. As soon as the cutscene ends STEP BACKWARDS, turn around, and sprint off the ledge to the right. Heal yourself because you will get hit anyway. If you dont step backward at that part but try and shoot or step to the side you WILL die. Get out the smart gun, and use it in very short bursts (or the Scope rifle so you can see them when they are invisible). You can kill each android with about 60 bullets from the smart gun, or 4 from the rifle - 2 in each leg. Keep your distance or their shotgun will rip you apart. Once they are dead replace the smart gun with a Pulse Rifle and Shotgun, stock up on ammo and pick up the grenades.
As you go up the stairs into the next room Weyland will come at you. Fire all 4 grenades at him on your left then run to your right and get the shotgun out. Play ring around the rosies (run around the statue basically so he cant hit you) and shoot him with the shotgun (double or single, doesnt matter) just dont get hit by him! He may run around back to where he was at the start so just be patient and keep popping shots at him. Once he's dead run up to the pad. As the next cut scene ends this part is crucial...either tap secondary fire so your gun hopefully recoils into his face or flick up your aim slightly over his face and press right trigger. If you succeed. WOOHOO, if not you'll have the joy of starting the mission again.
Hope this helps, and good luck.