Cheater in Grand Arena

Well, you're leaving out a lot of details that could help us answer that question, so I'm going to answer the best I can.

1) He scored more banners than you.
2) If you give us your SWGOH.GG profile, we could possibly take a peek at your roster and compare it to his.
3) We could also possibly go to your GAC Bracket and compare such things like Mods - as this probably boils down to Mods.
4) It may also come down to looking at what defenses you set compared to the defenses they set.