Earth Defense Force: Insect Armageddon

Very easy to get using a second controller.

Start up a survival game (any map) in two player and equip yourself with rockets and/or grenades. As soon as the game starts turn to the other player and rocket them to death.

Shouldn't take more than a minute or so to unlock. You can also revive them to get the revival achievement if you haven't got that one yet.

You can get this by killing any of the two teammates you have. (Look for the big green beacon identifying them) It's easier if the target is not a bot (co-op buddy, or another player over Live), as the bots will try to evade your fire a bit.

Also, when you revive someone, they're back at full health. So it is sometimes a good strategy to kill your partner and revive them. This has saved my ass a few times, either when the action is chaotic, or if were just setting up for the next checkpoint.