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, when you get low on health summon Bardle and spam Y to throw your Cymbal Monkeys to refill your gauge and summon Bardle again, at this point let yourself die and rinse and repeat.

This is the fastest of the "5000" achievements, I got this and the Cymbal Monkey achievement very close together.