Ricket's Special Delivery still hits for 5000 damage.
The Bolt Spitter hits for 700 damage (about 50 per bolt).
The rest deal insignificant amounts of damage (this also includes the Venture Co. Rocket Box).
EDIT: Was able to do double Boilburn just now. He moves to where the rocks crumble down so if you land the shot just as he gets there, he'll still spawn again, but the window to do it is really short.