Yeah this trophy is pretty tricky, but a lot of fun. here are some tips that might help you out.
Save points- There are the places where I saved at
1st save-mid chapter 5, the part where you enter the big room and a pregnant necro comes out along with 2 to 4 leapers. I'm glad I did save here because I died a ton on the next few rooms with the stalkers.
2nd save-Very end of chapter 9, right before you fight the tripods nest. You could maybe wait til chapter 10 to save if you wanted but i screwed up on this part once so i made sure i saved before it next time.
3rd save-beginning of chapter 13, after the drill ride
Weapons-
Pulse Rifle, this is IMO the best weapon in the game for killing necros, once you power it up and can do a lot of damage you can just keep firing even when you miss. I never really used the secondary fire on this or upgraded the secordary.
Detonator, I could do the stalker parts without this gun. It absolutely owns the stalkers which is one of the parts I had the most trouble with. You can also use it to cover your back in other parts where you get jumped. Make sure you set the mine instead of hitting enemies directly if you can. It does more damage if they run into the mine.
Contact beam, I only bought this near the end of chapter 13 and it was pretty much the only gun i used from there on. The last two chapters you will own with the secondary fire upgraded so you freeze your enemies. I just ran past a lot of enemies in chapter 15 thanks to this things.It also helps a ton with the last boss.
The evelator ride
There is a trick for this one, make sure you have a lot of statsis saved up, kill the first guy that drops, wait til there are two guys dropping and statsis them. Just keep statsising them for the whole ride. A third will drop, statsis him too. It's a long ride but you don't have to kill them, just freeze them.