ClusterluckG 30

You probably won't achieve this during casual play, so you'll have to do a little grinding for it, but it's not that difficult to do. Grenades, Boomshot, and the Hammer of Dawn will make this easier if you get clusters of enemies.

You can do this as early as Chapter 1, Act 1, which is the tutorial level. When the Grubs are plasma cutting through the door, wait until the door drops and three Grubs will enter the room. Toss a grenade at their feet and hopefully blow them all up at once; then rinse and repeat.