change the XP multiplier in mod config

It's a bit cheaty. But killing a boss with a long death animation like delirium and then exiting/resuming the game allows you to refight them and re-get the exp.

A less cheaty variant of this is... doing the same thing but different. A glowing hourglass or other teleport active can get you out of the room and refight the boss.

Something that might work too but I haven't tested yet is trying to use a glitched item that spawns tons of enemies and killing those repeatedly.