I'll have a go
Most have already added lots of good suffixes, but what about zooming in a degree to add flavor or just have a different menu selection?
Here's what I mean, instead of WyrmSlayer, what about ScaleBreaker? Sort of focusing on the how and not so much the sign on the office door.
Scale isn't specific to dragons, but the guy that told you his name can explain in the next breath that he killed a black dragon with a big hammer.
A vampire hunter could be simply "Staker" to get away from the two-word title, or "Sunlight" to show how dangerous he is to vampires and also with a bit of humor, or Fangbender/Fangturner if he's famous for seeming invulnerability to their bite, etc.
If you like the formula of [target] [hunter] then alternate [hunter] titles that haven't been said if irc could be... Scourge, Chaser, Eradicator, Remover, Stopper, Breaker, Trapper...
Also, you could identify the guy by the trophies he takes? Hydra heads, Vampire fangs, Dragon scales, Demon horns, whatever.
Or, you could tie him to a location, even using the usual titles. I think "Vorax, the Demon-Butcher of Ghazador" is more flavorful than "Demon Butcher" or the like. It simply forces the next question of "Why is he called that?" or "How did he get that name?"
Just trying to throw out different approaches for you...