You'll unlock the
Grenade at level 2, that's using it you will get this achievement early on in the game.
If you are a Titan,
your level 4 upgrade,
Fist of Havoc, releases a huge field of energy all around you and kills the weakest ennemies in one-hit.
If you are a Warlock,
your level 4 upgrade ,
Nova Bomb,hurls an explosive bolt of void, killing the weakest ennemies in one hit.
Wait until there are some bad guys packed and then toss your grenade (by pressing
) or release your "super-power" by hitting both
+
at the same time. There are plenty of spots giving you the opportunity of earning it eveywhere in the game.
I personnaly got this achievement on ennemies called
Thralls, these guys are melee and will rush at you. They are often always moving by packs of 4 or 5.
If you haven't got the achievement yet, you can get it further in the game very easily. It's of the very first mission of the Moon, by the end, where you have found a dead guardian; there will be a huge gate opening and dozens of Thralls coming to you. Enjoy !
These are Thralls: