Step 1: Easy Playthrough
The story mode is very linear and only really allows you to move forward so I wont be holding your hand through the entire game, but instead just highlighting when to do what at certain points.
So, we're playing on Easy difficulty first, this is because you need to play through on each difficulty once and it'll be easiest to work your way up to Hard mode, learning the level layouts/enemy locations etc.
On this level you can pick up 2 achievements. The first of which is story related and as such, you can't miss it. Progress through the level until you reach a motel looking building, you'll end up in a small room where you'll be able to see a pool outside. As you approach the pool an alien will jump out of it, kill it and kill the other alien standing on top of the motel slightly to the right and you'll net yourself the achievement!
While the next achievement is culmative you have ample opportunity to unlock it on the first level and given that the sniper rifle is probably the best weapon in the game I'd suggest you stick to using it through the entire game. If you don't unlock it during the first level, don't worry, there's plenty of time yet.
At the start of level 2 you'll be told to jump on the HMV turret, while you're on the turret there are plenty of enemies for you to pick up the next achievement, just keep the trigger held down and keep killing the enemies until you unlock this achievement.
Kill 15 enemies on the HMV turret
Another story progression achievement! Toward end of this level your AI partner will tell you to cover him while he assaults the gas station, as per my usual advice make sure you're using your sniper rifle and just pick off the aliens that are gathered around the gas station across the street. Once he's destroyed the tanker on the other side of the gas station the achievement will unlock.
After that you'll engage the alien aircraft. Once you've defeated it continue on and down the ladder making sure not to swap out the rocket launcer. As you continue toward the next building a few aliens will spawn, kill them with your assault rifle/sniper rifle, once you've killed them switch to your rocket launcer and get ready for the next achievement. So, when you turn left and move toward the alleyway 2 aliens are gonna spawn, you need to aim the rocket at the ground about half way in between them so that the explosion kills both of them. If you don't manage to get the achievement you can either try again on the next playthrough or restart from the last checkpoint and just try again.
Additional tip / alternative method from JMJimmy:
After destroying the second airship with the rocket launcher, keep it and when you're told to get in the Humvee, don't. Wait until a second enemy spawns up top and shoot the bricks in between them. If that doesn't work for you, kill the guys on the right hand side of the door and wait for a couple to run out, you'll get as many as 3 sitting behind the black sedan which you can kill with the rocket as well. They are a little harder to get though since the car explodes and that doesn't count as a rocket kill.
Right, last level now! You get this achievement towards the end of the level, once you've reached the part where you have to cover Nantz while he paints there enemy command center, cover him and it'll lead to a cut scene and the achievement will unlock. This is another story progression achievement and so you can't miss it.
You'll get the next achievement for completing the game. Nothing else I can really say!
Complete the game on Easy Difficulty
Back at main menu:
Once you've completed the game and the credits have finished you'll be back at the main menu, select "Unlockables" and then "Videos", just watch a video and the next achievement will unlock!
Step 2: Normal Playthrough
Second playthrough now! The next achievement isn't difficult but it does require you to complete a level without dying or restarting, you should try and get it on the first level because it's the easiest of the 3 levels. Play it safe, hang back and make use of the sniper rifle. Stay behind cover, if you get hit then hide and let yourself heal. If you're playing it safe then you shouldn't have any trouble getting the achievement!
Color Me Impressed
Complete a mission on Normal without dying, failing, restarting, or continuing campaign
All that's left now is to finish this playthrough and then play through on Hard mode! Just play through the remaining 2 levels and once you complete the game you'll unlock the next achievement!
Step 3: Hard playthrough
Now we're on the final playthrough, by now you should be familar with the game and be able to play it through on Hard mode knowing what to do. Upon completing the final level you'll unlock the last 2 achievements!
Congratulations, you should now have unlocked all the achievements!
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