The only thing, Heartless that are resistant to magic can also resist Light magic, unless Oner swings with his keyblade with the light infused in it. Because that still counts as a normal melee attack. And the Runemasters also can't take damage from the front. But you wrote it late so I can see you might have forgotten those things.
And that one ability where Oner released some magic with the reminder of his mana sounds a little bit like
Stun Impact. It is a pretty basic ability.