First I would recommend purchasing a LAN adapter if you haven't already (make sure to buy the nintendo official one, I've read that people have had issues with third party ones not working). This will clear up some of the issues you're having I think but there will still always be some degree of lag even if it's tiny. I've noticed that, since I use c-stick to tilt, that c-stick inputs are especially laggy or sometimes just don't register when I'm playing online even when it's a game that feels like a good connection. I will say, though, I do not believe I've ever experienced what you are saying, where it feels very laggy for me but the other player seems perfect, typically when it's a laggy game it very clearly is a laggy game for both players. But, as I said before, I do believe that even the most lagless online game still doesn't feel as good as playing offline so it's very possible that you are just experiencing the normal amount of lag that exists in all online games and you just aren't used to it as much as your opponent is.