Kill a Hunter with a melee attack (let him rush sidestep, then attack).
Kill a Golod Swordman with a melee attack (hit him with a pistol burst from very close, then immediately attack).
Run over enemies with a Warhog.
Blast groups of enemies with a Scorpion Cannon.
Skip a major part of level 5.
Shoot down a Banshee with a rocket launcher.
Shoot down a Banshee from a Banshee.
Finish the game in legendary mode.
Playing through the entire game co-op in legendary mode.
Taking out a friend (not in a vehicle) in multi-player with a plasma grenade.