I posted a while ago about burning out on eviternity 2 for a while after beating map24, the one with the giant warship
Went back to it and found a pretty cool custom boss fight on the next map that thoroughly impressed me.
I'm not gonna blogpost, but it was remarkably well designed, and the fight doesn't drag out, it's just difficult until you learn the quirks of the boss. You still have to deal with a mob of enemies accompanying him that he likes to resurrect, but it doesn't drag itself out or bullshit you.