Zoro santoryu to me seems like one of the unrealistic things that is most easy to do, just use it for finish moves, he dont need to use it mostly of the time.
The things i see as most difficult are the character designs of some strange character, like Pearl, Alvida, Kuro and some other and the body language and movements of Luffy & Buggy, not just because of the DF powers, but also because the two are very "gag character".
Arlong and the fishman shoud be quite easy if we see aliens in those super heros movies, they shoud give more budget to Arlong and Hachi, but the other dont need that much.