Easy. He took the Mobile feat and would atart his turn 30+ feet in the air. On his turn he would “stop flying” and fall on an enemy. The DM tried to calculate fall damage for him but forgot Monks negate a lot of fall damage. He would then make an attack as part of the fall dealing additiol damage followed by a Flurry of Blows to any other enemies within 5 feet. Then he would use his move to fly back up 30+ feet and didn’t provoke attacks of opportunity because of the Mobile feat. Then he just swatted arrows out of the sky when it wasn’t his turn.