It just seems he does such absurd damage to you that it's a race to put him down.
Mario Jumped and mallow defended unless he was healing mario or FP for more jumps.
To be clear when I said I was doing it kind of a low leveled it's not in a disciplined way or anything. Just not killing every enemy in sight like I normally. I'm at Belome 2 around level 11, and it's the first boss that I actually couldn't beat after two or three tries because he just spams party-wide sleep spells (decided not to use accessories because it felt too cheap at first but now it feels cheap for the boss!)
It wasn't a low level run but I've done a challenge where I didn't use any equipment or items. There's actually a boss later on that can completely lock you down if you have no status protection at all.