Crocodile

Crocodile ate one of the "Devil Fruits" available in his canon, the "Sand Sand Fruit" ("Suna suna no mi"), and has a variety of powers as a result of that. First- since the Sand Sand Fruit is a Logia type, he can literally be made of sand whenever he wants to. He's trained himself so that turning into sand is a thing he can do out of reflex, which means that, say, surprise cutting through his neck...won't actually do anything harm to him, the cut will just pass through sand, and he can just reattach his head. (Nice try, Doflamingo.)

It doesn't end there. Crocodile can create blades of sand with serious cutting power out of his body, can create sandstorms, can suck all the water out of your body, and can suck all the water out of an area of land and cause centuries worth of erosion in a minute (handy for breaking out of prison!). If he's on a landscape made of sand, he can sense underground streams and do things like create pits of quicksand, he can slice the landscape apart, and he can use the sand around him. (You really do not want to go up against him in a desert. Really.) His power does have a serious weakness, but you pretty much have to find it out by accident while fighting him. This method is not recommended. (And even after you find out his weakness, it's likely to take more than one fight to beat him, and unless you're a shonen determinator with crazy luck, or very well prepared, you'll probably be too dead for round two.)

If all that wasn't enough, he's also just strong. That hook he has replacing his left hand? Yeah he can stab people clean through with it.
And he has a poisoned hook under it.
And he has a knife under that.

It is definitely possible to beat him, though- he's been beaten by a number of people in canon (gdit shonen determinators). And I know there are people on the community who could overpower him.

Of course, if you're not fighting him none of this should come up. But Crocodile can...kind of piss people off. If you think there's a chance of your character and Crocodile having an antagonistic encounter, or if you'd rather not have your characters interact with him, or any similar issues, please comment here and we can make things work!