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 inside the 2nd group of enemies, summon Biggun, wait for his ground slam then spam Y to throw your grenades if you need that achievement and wait for yourself to die then rinse and repeat.

It looks like Biggun doesn't do anything in terms of absorbing damage but keep at it, it only takes around 10 runs to pop, the grenade achievement took roughly twice as long.