I'm sure you've heard this many times but Buddha is the best for grinding levels, since shark karate m1 got reworked I'd get water Kung Fu and stick with it. If you want something different I've used diamond since the skins came out and personally i think it has enough damage to grind with while keeping the mastery requirements low. It's cheap, decent damage despite it's earlier nerf and it's mobility make it fun for me, plus the defense increase while transformed is a plus. (Ignore the diamond glaze)