"The fire guardian made several mistakes because of you. Didn't you notice that?
"First of all, he was distracted during the battle because he wanted to take you in as his pupil. He stopped attacking Himmel Soan, giving him a chance to get used to Inferno. By the time he realized it, Himmel Soan was already able to withstand the saber.
"Secondly, after defeating Himmel Soan, the fire guardian was busy stopping you from killing yourself rather than finishing his opponent off. It gave Himmel Soan another chance to get used to the flames. Moreover, he enhanced himself during this process. Do you think that's fair?"
Leela had seen everything.
Indeed, if it hadn't been for Firey, the fire guardian would have had several opportunities to kill Himmel Soan once and for all. He wouldn't have had a chance to recover or strike back.