Add a frustration counter to certain boss monsters, which increments every time they want to close to melee distance with you but are unable to. Once this gets sufficiently high, they become capable of dispatching or shoving aside monsters (even if they aren’t normally) and breaking boulders to get to you. Different bosses have different thresholds for when this happens.

This prevents you from trivially using a boulder fort to shut down bosses like Master Kaen while you slowly whittle them away with ranged attacks, but allows you to employ strategies like that for a limited period of time.