A few possible ideas:
Remove the “Nature only” defense from caster and nearby, making it a defense from all sources instead.
Caster and nearby allies gain 33% mana generation for 3 turns.
Caster and nearby allies gain 33% Crit for 3 turns.
Caster and nearby allies are immune to any new status effects for 3 turns. (Or all special skills instead)
Or simply add an attack boost like Kiril.
Alot of other good ideas in earlier posts too.
He definitely needs a boost to balance better with other HotMs.