There's Loot Aboot!

There's Loot Aboot! is one of the Challenges available in The Arena.

Description
"Pierce is refusing to come to the next battle unless you make it worth his while. Gather up as much as you can, before he up and leaves!"

Strategy
You'll definitely need Pierce in your party. Repeatedly spam his Mug ability as many times as you can, make sure he has Pick Pocket equipped. It's also recommended to have Gruver and Sara in your party, in order to decrease spell cooldowns and the number of beats required to cast a spell, so more casts of Mug can be executed.