It seems a bit strange to insert a training arc when nothing so far has even come close to matching him. Yeah, Arlong is supposed to be powerful, but from a narrative point of view training arcs are usually done so you can hype up a villain as being 10x stronger than the one the hero just barely beat, and then have the hero come back and beat them without it seeming like the villain wasn't actually more powerful at all. But here we just have no actual reference to what a tough battle would be like anyway for Cain, so it just seems unnecessary.