Army of Ruin

As the description says, this is cumulative across all runs of all stages, whether you win or die. Make it a point to use it regularly (I routinely forgot, or I'd always save it in case I needed it but then I'd never need it). There's really no quick way to grind this out since you're forced to wait for the cooldown no matter who you play as, so just make a conscious effort during your time with this game and you'll get through this before finishing everything else.