You may have just stumbled upon a circumstance that would be well served by a gentleman's agreement. He believes your specialization in one character makes the game less fun for him, and perhaps you would benefit from playing a wider variety of characters. Try playing on Random or make it a rule that if you use a character you can't play him again for the next two matches (for example, you could play A, B, C, A, B, C, etc. so you still get to play Link regularly).
Similarly, trolling behavior like getting everyone to gang up on one player or exclusively fighting one player when there's no team reasoning for it, makes the game less fun for everyone. Your group should also agree that they shouldn't do that kind of thing. Probably better to not try to come up with rules for this since it's so subjective. If the game allows teams, perhaps rotate teams so he has to be allied with you sometimes and must attack you other times.
Remember that, while trying to win makes the game interesting, it's more important that the game is fun. If he's winning more than you are and still not having fun, clearly winning isn't all he cares about. It's boring to fight the same opponent over and over. Imagine how you would feel if he also always chose the same guy and spammed certain attacks. It would get kinda old, right? But even if you think that would be fine, maybe he has different ideas about what entertains him.