Well, fire would actually work against anything that regenerates in theory since it would kill the new flesh as it regenerates, so long as the fire does more damage than the creature can regenerate. However, I somehow doubt that the demon thing would care about being on fire. 

Also, not sure if I brought this up, but my character name is Natus, not Natsu. One of my tricks for creating names is just translating word into Latin, and his name translates into "Rhythm End."