HOW TO KILL: Hiei Jaganshi
DO NOT ATTEMPT! Trying to kill this particular fire demon will only result in a fist to the face, a sword in your side, and a dragon down the throat, all of which may individually result in death.
If you are depressed and ready to commit suicide but don't have the guts to do it yourself, either:
Kill his sister, Yukina or
Tell Kuwabara that Yukina is his sister.