The Last Kids on Earth and the Staff of Doom

A good place to farm this is in the building to the top left of area 4.1, there's a large group of enemies right by the entrance. Remove your armour, run to the 2nd group of enemies, spam Y if you need Dirk's grenade achievement, and summon Skaelka, use a few basic attacks to refill your gauge wait for yourself to die then rinse and repeat.

This was the grindiest achievement in the game for me as I used Dirk throughout the entire game and only got it to pop shortly after completing everything else in the game despite using it during every large group of enemies I came across, for this reason I recommend using Dirk as much as possible throughout the game as there aren't as many good opportunities to use the other character's abilities to full effect.