Suggestion:
Make flying bosses have the flying attribute like regular monsters. In turn, remove the part of Heal/Cure/other tile spells that prevent them from hitting flying monsters.
Reasoning:
While the interaction between flying/tiles is cool on a thematic level, Archers are consistently worse against bosses due to their inability to get damage bonuses from their weapon/skills. This is of course solved against regular monsters with their mastery skills Ignore Defense and Anti-Air, but the latter is useless against bosses. Implementing this change without letting tile spells hit would decimate non-melee Clerics early on.
Balance unfortunately must come before flair. Besides, other spells already hit flying spells even when they don’t make sense (Earthquake for example)!