I think he should be able to be beat.

There are many good ideas as to why he's doing well, and it could be a combination of all of them.
As it's Level 3, he will have had time to assign points on his players that way he wants, and this can make a 3 star play like a 5 with the right formation and orders. You could look at his players' attributes and see what he has done there. You can't see his orders and arrows though which could of course be part of that.

Put your best defensive formation out and try to be at the match to make changes.