Demons Souls with only your fists. You are only allowed 5 deaths

Gta4 one is actually pretty easy. Just gear up and go to hospital, start shooting and when the cops show up go to that long hallway and just keep your ass covered while shooting cops as they pop out of the cover. It's pretty much impossible to get out alive though.

MGS4. Big Boss emblem. Currently doing that. Want the corpse face camo.

The Boss Extreme Difficulty (Hardest)
No kills
No alerts
No deaths
No recovery items
No special items
Complete under 4 hours

Wish me luck.

Killzone 2: Beat it on Elite (Warning General Raedec room, will give you hell)
WarHawk: Just trying to get the platinum trophy..... Thats all that need to be said

The Mile High Club from COD4 on veteran is surprisingly easy. Once you get the enemy pattern and how to move it's not that bad. My first attempt took 1.5 hours, and the second time it took 15 minutes.

Half Life 2 Episode 2 'Little Rocket Man' achievement springs to mind.

Finish FFIX at Level 1 or finish the game under 12hours.

Final Fantasy XIII NCU (No Crystarium Usage)
It IS possible, give it a try if you need that extra push towards suicide

edit @ below
Dark Souls SL1 is very possible thanks to Lightning Weapons, I could finish it right now but Im leaving it as is so I can Darkwraith.
Using Iron Flesh will pretty much get you killed at such a low level anyway, especially if you're thinking about using it against Four Kings

Dark Souls, beat the game without leveling up. (Wonder if it's still feasible now that Iron Flesh and the pyro glove have been nerfed...)

Dead space 2 on hardcore seems pretty hard, no checkpoints & you can only save 3 times.. Ain't attempted it yet & don't think I will.

Gta 6 stars isn't an impossible feet but the multiplayer trophies in gta4 are absolutely ridiculous. The trophy in red dead redemption where you have to keep the same horse for 20 missions was pretty frustrating & hard

wipEout HD/Fury: "Beat Zico" trophy & "Zone Zeus" trophy. those two trophies require absolute perfection. no luck. no cheats. just pure human perfection.
then there's the "Airbraker" trophy. and that's only a silver trophy. i still think that game's platinum trophy is by far the hardest platinum trophy currently available.

for me at least, the "Supersoldier" trophy on Crysis 2 is insanely difficult. beating the game on supersoldier requires so much patience it's unreal.