Greatest Jubilee and Orologions.

[Crimduval]

The Jubilee spawns so many light sources with enough luck that you can get a contant supply of Orologions (the pickup that freezes everything). This really slows runs down as it causes mobs to spawn a lot slower and seems to bug some up. Been fighting the 25min mark crab boss on the Tower for close to 20mins on an endless run. In that time, gained around 7 levels.

[Falcon]

I think the crab boss's HP scales with your level, so that might have something to do with it?