Titles are better of being descriptive than vague imo. Also:
Demon Hunter is a class. You sure could use it as a title, or you could substitute it for something else, but at the end of the day most people will look at your character and go: it looks like a duck, talks like a duck, kills demons like a duck, so it’s a duck (duck = demon hunter).
A highly experienced warlock or a powerful mage is, in this setting, extremely dangerous. Like stopping swathes of minion-level creatures in their tracks with the flick of a wrist kind of dangerous.
I think if you’re concerned with playing an OP demon hunter, just… don’t RP an OP demon hunter? Honestly, you shouldn’t be beholden to nor give much thought to the people that RP a concept ‘poorly’, and there’s no need to break your back over figuring out a funky + vague title to describe your character when some very on-point titles (Illidari, Demon Hunter, etc. etc.) already exist.
edit: hope the tone of this reply doesn’t come across as condescending or anything, just wanted to give my $0.02 without the fluff