I suppose if you want a reasonable nerf for Turtles, you can always increase the cast time of Giant Kamehameha from 1 sec to 2 sec. Giant Kamehameha is ridiculously powerful, so it would make sense for it to require more time to charge. In a gameplay perspective, this would make using Giant Kame require better timing and strategic use instead of just firing it willy nilly. Obviously the increase charge time wouldn't have much (if any) of an impact in PvE. As for compensation to the skill, you can always:
a) Give it Knockdown RP effect like Super Kamehameha
b) Slightly reduce CD
c) Give it Cast Time Decrease RP effect to somewhat offset the nerf
Those are my two cents though.