... That's the joke. Meditation is based off of the move from Dragon Quest, where Final Bosses could use it and heal 500 HP at once. That's why Alice acts so incredulously when Luka can do it, because it's not something he should just be able to do (but can, because he's part fallen angel, and loosely references how he's related to the final boss).It shouldn't even appear, it's a field-only spell (seriously, who can meditate during a fight?).
(I know that's from Final Fantasy I, I'm just including it here to illustrate the point; being able to heal yourself to max health is indeed hax)
Beyond that, this is a turn-based RPG game where a monster girl will very casually walk over to Luka, give him a handy/blowie/kiss/puff-puff/etc, and he just stands there without thinking to dodge unless he's got Sylph or Undine to help him out (not to mention all those battlefuck games where that's the main combat style). Healing half his health for a turn isn't that much of a stretch.