For those who prefer a video guide, this shows beating the Crab boss:
The idea is load up on Frankenimps, then go into the pit where he is. Wait for him to charge, if his arm is up while charging, throw your imps at him & then when stunned attack. Before he comes to, recall your imps.
If his arm is *down* while charging, then move out of the way & wait for the next charge.
Then just rinse & repeat until done.