You only need to master KOTR once. Then you can get a master summon, plus the master summon you get from killing Emerald Weapon, and boom, everyone can cast it.
EDIT:
you people suck, I got through TMNT the arcade game on NES on two lives, and Baxter Stockman was easy as hell.
EDIT 2: I just noticed UPC was the one who had trouble with Stockman so I'm laughing at him now.