Warhammer 40,000: Shootas, Blood & Teef

This achievement is very easy at the start of the game (level is Crash Landing). You quickly reach a spot through the tutorial in which you're given free grenades and can throw them at the Gretchin/Grots/Goblins below.

The checkpoint is just before it so throw the grenade, kill them and reload until you have it. Video below for the spot (credit to Dreemax for the video).