imo without the logs and you not showing your defense he seems like he could be legit. He has 3 GLs to your 1 from what others said so he has at least 2 battles where he can get better banners from a solo with each GL (2 battles because JML is seen on defense). Plus id assume he is good with counters to not fail battles, which either meant just a weak defense by you, or just he had excellent mods/strategy.
also just because someone has a lower GP doesn't mean they cheat, it more likely means they are just good at the game. just like how you winning against people with 3+ GLs to your 1 GL means you're good at the game, not that you're a cheater.