If you wondering why I "haven't yet won" this achievement, I have. I rarely go on XBL though.
Hardcore is not difficult at all gameplay wise. The key is to treat your playthrough the way you treat combat: stay calm, never rush things, and do the same thing you have practiced before. Sloppiness, nervousness and careless mistakes are what get you killed on this mode.
-There are no definite points on when to save. Some people breeze through fights that others struggle badly. I saved ahead of sequences where I had trouble in the past.
-Always keep full health at all times. Even if you are only missing a few blocks off the top, fill up. There is nothing worse than getting caught off guard and getting killed when you had a medpack sitting uselessly in inventory or the safe.
-Stay calm and play as you normally would. I cannot emphasize this enough. Combat difficulty in hardcore is equivalent to Survivalist. It is NOT that hard at all.
-Be familiar from previous playthroughs of where items are, and most importantly, where enemies come from.
The following is a description of my playthrough accompanied by brief tips on major fights by chapter.
Loadout: I recommend Javelin Gun, Ripper, and Contact Beam. Buy the Javelin as soon as it becomes available and upgrade the damage to the full 100 pts ASAP. Ignore everything else upgrade-wise until that is accomplished. Javelin ammo is dirt cheap and with full DMG upgrade you will be able to kill most necros in 1 shot from range, enhanced necros in 2, and pack a far bigger punch than the Cutter can provide. Efficiency wise, 400 credits for 2 spears (2 kills) is absurd, especially once you get the Vintage suit's discount. Once the Ripper becomes available, replace the Cutter with it. The Ripper turns the Pack into a joke and also tears through normal necros with extreme efficiency. Even with no upgrades whatsoever, you can kill a black slasher with a single blade. The alt-fire will also form a nice backup capability to your javelin. Finally the contact beam is there for nothing other than the alt-fire, which you should upgrade along with the Special upgrade (Stasis effect on alt fire) asap. This will kill and/or stasis everything in a large radius around you and unlike other weapons the animation cannot be interrupted by enemy attacks. It will save your ass many times especially on some of the more difficult close quarters fights. Learn to use all three of these weapons proficiently. I carried 50-70 Javelin ammo with it as my main weapon, 10-15 Ripper ammo as a close range weapon and situational backup, and ~10 contact ammo as a panic button capability.
Upgrades: First upgrade for hardcore *must* be the first two RIG HP nodes. I don't see how any other choice is justified. You immediately add +50% to your health, using only 3 nodes, on a mode where you must not die. After that, focus on maxing out Javelin DMG. After that, finish maxing out the other RIG HP nodes. After that, upgrade your Stasis and Javelin as you see fit. Once you get the Contact Beam, max out the ALT path and the Special. You won't need to upgrade the reload or capacity nodes until the endgame when you will have plenty of nodes anyway.
Major fights by chapter.
This should be cake. You know the drill, scrounge for every pickup, use Stasis/TK to save ammo, etc.
Hanging from the train: The first real test. The main way to die is letting an exploder get too close. Keep the cutter in horizontal and the instant necros appear take off their legs and an arm, this should kill them. Always kill exploders the instant you detect them. Do not forget stasis when you are being swarmed. Remember that TK spears/blades might not kill a necro outright but will knock him on his ass to buy you a few precious seconds. At the end when the train is about to fall, stay calm and fire one javelin into the weakspot, and you are home free.
Sidenote: I don't remember if this is the chapter, but in the first hallway where you meet exploders (With the commercial repeating "TST") it is VERY easy to die if you are inattentive. One exploder loves to come out of the wall vent in the middle of the hallway and sneak up while you are distracted by those spawning at the end of the hallway. Constantly watch your surroundings. If you turn around and he is right there, Stasis him immediately and pray it gets him before he detonates.
Not much happening here, should be easy.
Winding staircase leading down to the church library: Lots of necros here. Best to run down, trigger them, then run back up and gun them down as they rush you. This is a great place to save ammo by impaling one to a wall with javelins, TK'ing his arm blades to kill his friends, repeat.
First stalker encounter: Stand next to the door, nail them with the javelin as they come. You should have the Javelin DMG fully upgraded by now; if not it may take 2 shots to kill them.
Dragging scene in the chapel: Stay calm and one javelin to his weak spot will end it. Immediately run back to the little hallway you entered the chapel from. All the Pack will funnel in through the tiny doorway making it easy to slaughter them. Be wary that one slasher spawns with them, he likes to wait near the doorway and catch you unawares as you exit. If you don't have full health his "grab" QTE can kill you outright.
Huge fight against Pregnants/leapers, ground floor of the apartment complex: I used my first save immediately before ths fight. For the pregnants, Stasis then Ripper alt-fire to take off the legs and an arm, this usually kills them. If you mess up and they pop, shoot a javelin at the ground next to them and electrify it. This should kill all the critters. Your main threat is the leapers. Whenever you get a clear shot to stasis one, do it then kill him ASAP. Do not get cornered as they will easily leap over your fire and thrash the hell out of you. Try to catch them on the ground (e.g. coming around a corner where they can't leap) then Stasis and open fire. Ripper up close annihilates them, tear off their arms and head and they're done.
Hallway leading to the school: Watch out for the pukers that spawn from behind you as you approach the door.
Gym fight: Can be tricky if you panic. Stand up on the stage with the ripper and all the Pack will run right into it. Use ripper alt-fire or the Javelin to kill the exploding babies from range. Again remember your stasis and the area-effect of the javelin alt-fire if you mess up. Last but not least, stomp the corpses on the gym floor before the fight, or an infector will turn them into slashers. If the bodies are stomped, the infector will attack you instead. Make sure you take him out or he will run up and cause that hugely irritating QTE that lasts 15 freaking seconds where you have to tear it off your face.
Brute at the transit station: Immediately stasis him. 3-5 javelins to his weak spot will take off an arm. Repeat until he dies. If the javelins are bouncing off the armor you are missing the spot. Do NOT let him get close.
Guardian: For all guardians, stasis then cut the tentacles using ripper alt fire. Or simply shoot a javelin and electrify it, 1 or 2 uses of this will kill him. Never walk up as he has an automatic-kill attack even if stasis'd.
Tripod elevator ride to the solar array: Bring extra health in case. Stand exactly in the middle of the elevator and do not move, only rotate the camera to aim. One javelin to the weak spot will kill each tripod. Just keep spinning, aiming, and firing. Heal immediately if you get hit.
Cysts: Starting in this chapter they become more common. Don't get sloppy and run right into one. Be patient, it is easy to forget where they are.
Room before the mainframe/AI section: This is the room with the decompression windows and trip mines. Stay in the previous hallway and shoot out the left window from there. Wait for it to seal itself then you can enter without fear of a cheap death. The main threat is the exploding babies that come from two sides.
Ending fight at the CEC door: Bring lots of health. I stood right at the door. You will fight Pack, pukers, leapers, and slashers coming from your 12 and 3. Kill pukers and any other non-Pack necros you spot from long range with the javelin as soon as they appear. Use the ripper to tear through Pack and any slashers that get up close. If you get overwhelmed, pull out the contact beam and alt-fire immediately to kill everything around you. Again, do not forget your stasis. Necros will try to gang rape you at the door which means 1 or 2 Stasis shots should freeze all of them and buy you a few seconds. The key to this fight is killing enemies before they get close.
Zero-G with the Nest: I sniped him from range with the javelin. One shot to each sack should burst it. If you have the Javelin Special upgrade with the explosive effect, just shoot the main body and detonate. Be careful he doesn't hit you and knock you back into a flame ring.
Stalker encounter: Same as before. Tease them out then javelin. The main worry is the exploders. Stay away from the vents and exploit the Stasis canisters on the ground to freeze them immediately when you spot them. A useful tactic with them also is to quickly shoot a Stasis at the ground next to them when they poke their heads out right before attacking you. Proceed to headshot them with the javelin for an easy kill.
Tripod nest: Six gas cans will blow up the bottom tank. Once you take the elevator down, TK the first two cans to you and toss them. Tripod #1 shows up. Stasis and one javelin to his arm weak spot will take that arm off (I freaking love that gun). This slows him down, now simply run in a circle until the gas cans respawn. Stasis him, toss them, repeat.
Long dark engineering corridor: Use TK on the pile of crates from a distance to make the brute come out early. Same as before, stasis and javelin his weak spot. Same for the 2nd brute. When the regular necros start spawning, quickly run all the way back to the entrance then pick them all off as they run towards you. This should be a very easy encounter.
Two-way hallway leading to the centrifuge: This is the hallway with a node-box at the left side end, and the centrifuge objective at the right side end. Lots of enemies here. Poke your head in from the entrance and snipe bitches with the javelin until you can't tease them out anymore. Your call whether to go for the node. Either way, watch your back constantly en route to the objective as they love spawning from behind.
Decontamination room: Pack will appear first from both ends of the room, massacre them with the ripper. There will be a very short lull then black slashers and black pukers arrive. You should have the Contact Beam alt-fire and special upgrade all upgraded now. Stand in the middle next to the table, use it and watch them all disintegrate.
Cryo storage room: Black slashers and exploders. Again the exploders are priority #1. Be ready as they attack again after you do the hack panel. Remember that you don't need to aim for the exploder's sack, a javelin shot to the body will not only kill him but send him flying away from you.
Captain's nest: Some of the stalkers here may be black, so they may need two javelin shots. Stasis stalkers here to be safe.
Dark downslope area with stalkers: Same tactics as before. Watch out for the exploding babies that spawn from behind when you approach the bottom.
Elevator ride: Slashers and black slashers will drop 2 at a time. Stand in the middle of the elevator and Contact Beam alt fire once they drop. Watch the body parts fly. Take around 15 contact ammo just to be safe.
Tip: Once she starts warming up the drill, run over to the box where you got the fresh battery. Pack will appear and run towards you but are either retarded and get killed by the drill, or trickle towards you 1 or 2 at a time. You can kill them by throwing random objects/body parts at them or just use the ripper. You can pile up a decent amount of cash and ammo by exploiting this (They spawn for a good while) and it is extremely easy.
Drill ride: Bring extra health. Stand right in front of the driver booth where she is. Most necros will climb up next to you, a few on the other side. I used contact alt-fire for those next to me and Javelin for the far side ones. The dangerous part is having them climb up when you are already taking fire from the lurkers. Hold down sprint and just juke around randomly to avoid their fire. You can try to throw gas tanks or shoot them but there are so many that standing still to kill one gets you nailed by 3 or 4 others, not worth it.
Government sector door: Do NOT try to hold them off next to the hack panel. This is a massive wave of necros with pukers, lurkers, slashers, exploders and leapers, many of the black variety. Best to snipe them from the hallway you enter from to trigger them to attack you, then methodically blast your way through all of them. There are two separate waves and they will be pissed. Watch for exploders as usual. Keep in mind you can kill the first wave, then leave and backtrack to the store before coming back. The second wave will spawn when you enter again.
Lab with the spinning lasers: Don't.mess.up. This section is not hard at all combat-wise. Remember you will not be able to outrun the lasers if you are slowed down from having low health.
Red-light rooms: Don't bother fighting all the way through. Sprint for the exits, stasis anyone who bursts out, trigger the door and walk backwards through it to ensure nobody runs up on you. They won't follow you through. Repeat.
Black brute: He will trigger once you approach the hack panel. Same MO as usual except he has more health, therefore I used contact beam primary fire. The stasis canister on the ground should keep him under long enough to get in 3-4 shots on the weak spot, this should easily kill him.
Eye poke: Don't mess up. Save beforehand if you frequently make mistakes here. Keep in mind the computer readout lets you know how close you are. The needle goes in at "0.0".
Regenerator: Stand at the hack panel and wait for him to appear. When he does, stasis, blast him apart (Contact primary or Javelin secondary), start the hack. By the time you finish he is probably healed and coming at you. Take him apart and stasis again.
For the remainder of the level simply run through as before, you should know the layout. Ignore any items/enemies unless directly in your way. In the room where you need the door to scan the corpse, just TK the body and toss it at the door. The regenerator will come in, run up and Contact alt-fire to blow off his limbs and stasis him, proceed through the door and keep running.
Stock up on health and contact energy. You don't need much ripper or javelin ammo anymore so sell them if you need $ although you shouldn't.
At the very beginning as you come outside kill the black leapers and make sure to snipe the black puker far off on the elevated walkway. He loves to use the slow-down vomit on you right as the regenerator makes his entrance. However he won't spawn until you advance to the intersection.
Same as before, run through and use contact alt-fire when needed. Heal whenever you get hit no matter how minor.
The final boss is absolute cake with the contact beam. Two primary shots to Nicole and one shot to the Market heart. Repeat 3 times and done.
I hope this helps anybody who has an obsessive need to beat this mode.