Would recommend waiting to do this until after you get the upgrade for Wolfimpa's (one of the middle treasure chests IIRC). Doesn't make it any easier, just a little shorter & thus less tedious.
Reduce your group to just a single Wolfimpa, find a lonely crab, and fire that imp at the crab. Let him hit it once, maybe twice if you're feeling brave, then make it retreat.
Back away, and rinse and repeat. Video showing it: