I haven't used MV, but I assume it works basically the same as VX Ace. As I understand it, RPG Maker doesn't really distinguish between 'damage' and 'healing'. If an ability 'heals', all that means is the damage is negative (so when you set a skill to "HP Recover", RPG Maker multiplies the result of the skill's damage formula by -1). When damage calculation occurs, RPG Maker just checks if the target gained or lost HP, and then gives the player the appropriate message ("(Actor) took 20 damage", "(Actor) regained 20HP", etc) based on that.
Let's say I have a spell that heals the target for 50HP. Because I'm extremely creative, let's call this spell 'Cure'.
- If I cast Cure on a completely ordinary target, it regains 50HP.
- If I cast Cure on an enemy that has Shell or something, so it takes half magic damage, it regains 25HP.
- If I for some reason make a fire-element spell called Flame Cure or something, and cast it on a lava golem that takes 50% fire damage, it will regain 25HP.
- If I cast Flame Cure on an ice beetle that takes 200% fire damage, it will regain 100HP.
- If I cast Flame Cure on a fire elemental, which 100% absorbs fire damage (can MV do that natively? you need a script to do that in Ace), it will take 50 damage.
Basically, if you make a 'heal' element, undead enemies will react to it exactly the way you want them to. For myself, I also like that the damage rate parameters don't distinguish between damage and healing, because it gives a Shell a slight drawback. People have also made various 'undead HP'-type scripts which just cause all healing to become damage, rather than rely on elemental affinities.
Going the elemental route means you can easily make abilities that damage living targets but heal undead ones, which is also neat.