Having a skeleton as the big wall to surmount after aquiring death magic isn't a good choice if I'm being honest
Could've been much better with literally any other big bad monster aside from a skeleton or zombie or anything that's already dead
in a way it DOES make sense to use an undead though... his magic is INSTANT DEATH, which makes it pretty OP again.... well ANYTHING living... which then leaves the giant question hanging in the air of 'but is that all it works on?', in almost any fantasy setting the monsters can, largely, fall into 1 of 4 categories, Creature, Undead, Spirit/Elemental, and Construct... his ability is GUARANTEED to work on 1/4 of these, but it's important to know whether or not it can work on the others.
fighting the skeleton shows he's effective against at least 2 of the 4 categories, and it's one of the two that are most likely to cause 'issues' for his skill set, and it'll be pretty safe to assume 3 out of 4 when he goes against a Spirit or Elemental, as they tend to be a form of 'concentrated life force', after this, to establish the fact it's a truly OP ability (the whole point of the series) he'd need to test it on a golem or something, to help fully define the limitations and defined use of the word 'death'