Yeah, the friendly Yan is the most difficult enemy in the whole game; I actually learned that the hard way during one of my early playthroughs. I had saved before doing the friendly monsters sidequest, so I could try fighting Ozma both with and without the bonuses you get from giving them the gems and stuff. After beating him without the bonuses I reloaded my save and when I got to Yan I realized that I didn't have the right gem to give him for whatever reason, so I decided to fight him and got my characters wiped out ;_;