It's the last Achievement I have left to unlock, which is to beat the game on Impossible. I'm on Chapter 7 now, and I haven't been having much trouble so far. I thought for sure the ADS shootout in Chapter 4 was going to be a brick wall, but not so much. The Leviathan boss fight took significantly longer, but it wasn't hard or anything. Also, when you return to the Medical lab in Chapter 5, and you have to survive locked in the ER with the Regenerator and some other necros, I thought for sure I was done for. I made it out BARELY. I have made some notes though:

• Necromorphs take about three to four plasma cutter shots to dismember one limb. You might want to sink more power nodes into weapon upgrades. After all, they can't hit you if you kill them first.
• From the beginning, just the basic Slicer necros take away about two health bars, out of four. 
• Stasis is your best friend. USE IT. Always keep at least one stasis pack on you at all times.
• Try to keep at least two medium med kits or one large med kit on you at all times.
• The necromorphs with the exploding arm tumors WILL kill you, no matter how much health you have. I'm wearing the Scorpion suit (call me a pansy if you must) and it's fully-upgraded in terms of health, and it still killed me. Take those fuckers out first.
• Try your best not to pop the pregnant necromorphs. If you do, use the Line Gun's mine ability or the Force gun to take them all out at once. If you don't have either, just rake through them with the Ripper. You don't want those things to get on you, they'll tear you apart.
• Take the bats out first. The fewer infected corpses to deal with, the better.
• The regenerator necromorph isn't much harder. Just dismember him and stasis him while he's down, and use that time to run away. Pretty much the same as on medium.

good tips matey :) 

Thats a good'un.

well its a platinum trophy on ps3, can't wait to get my first one

^^^
I will be going for a platinum in this game. Got 60 odd percent. I am planning to get all the trophies apart from Epic Tier 3 on my next playthrough. Then playthrough it on Impossible.
I figure that I wont be able buy powernodes at all on Impossible because that money will be used for med packs and ammo. But who knows.

I still haven't gone back to Dead Space and finished out the achievements I have left on it.  I've got 6, I believe.

I'm gonna finish my current Hard run, get Full Upgrades and be a pansy and use the bug to start on Impossible with the Military Suit.