Beat the game on any difficulty and you will unlock the impossible mode. You must beat it on impossible to receive this achievement. Like the name implies it is VERY hard. Always make use of every possible item you find. If you can't carry anymore head back to the bank to make a spot for it do not waste med packs. Sell air canisters, upgrade only what you need. IE, suit HP and Air and the only gun you use. Save as often as possible. Conserve as much ammo as possible, try shooting out the legs and crushing their heads with your foot to save ammo. Use the stasis and the stasis cheat unless you don't want to cheat. Its up to you. It does not effect achievements.

YOU WILL UNLOCK IMPOSSIBLE MODE IF YOU BEAT EASY, AS SOON AS YOU START THE GAME ON IMPOSSIBLE SAVE OTHERWISE IT WILL DISAPPEAR AND YOU WILL NEED TO BEAT THE GAME ON ANOTHER DIFFICULTY TO UNLOCK IT AGAIN.