Take out an enemy that's investigating an enemy body.
Bait an enemy by talking nonsense, then take them out.
Bait an enemy with your microphone, then take them out.
Choke an enemy out.
Knock a startled enemy out
Knock an enemy out with a door bash.
Kill someone with the shrapnel from a frag grenade.
Kill 3 enemies with guns under 5 seconds.
Kill 6 enemies with guns in under 5 seconds.
Kill 5 enemies with a shotgun in 10 seconds.
Kill 3 (or more) enemies with a single frag grenade.
Kill 5 (or more) enemies with a single frag grenade.
Kill 8 (or more) enemies with a single frag grenade.
Have a flashbanged enemy accidentally kill his own friend.
Neutralize someone while they are under the effect of a flashbang.
Kill yourself with your own frag grenade.
Have your frag grenade shrapnel kill you.
Over-cook a frag grenade.
Over-cook a flash grenade.
Get killed by a door bash.
Complete a level without being detected.
Complete a level without killing anyone.
Complete a level whilst using naught but the Taser.
Complete a level unarmed.
Complete a level whilst using naught but the Tranquilizer Rifle.
Finish 'Abandoned Complex' without killing anyone.
Complete a level without taking any damage.
Complete a level without saving once.
Kill the Degenerate.
Spare the Baker.
Kill the Baker.
Finish the campaign with the Baker spared.
Finish the campaign with the Baker killed.
Finish the campaign without using any weapons.
Complete the game without killing a single enemy, unless when forced to.
Kill everyone throughout the campaign.
Complete the game on the 'Easy' difficulty.
Complete the game on the 'True' difficulty.
Complete the game with ammo auto-grab disabled.
Complete the level "Steve's House" without firing a single shot.
Unlock all unique weapons.
You must really enjoy pain. Finish the game on 'Masochist' difficulty.
Pass out from drinking too much.
You really enjoy listening in on private matters, don't you?
Obtain a 100% accuracy rating with 40 or more shots fired.