I replayed one of my old saves today and it would seem that the best party setup I had was Zidane, Garnet, Quina, Steiner. It was the only way I could beat the friendly Yan, with everyone around level 90.
Try fighting the friendly Yan sometime. It has 65535 HP, and it has a defense/evade/mag. defense/mag. evade of 255 each. Not only does it counter every single attack whether it's physical or magic(provided you can even hit the thing) with the most powerful physical attack in the game, but it is also the only friendly monster that will spontaneously attack if you do not give it what it wants quickly enough - nor will it flee if you attack it. And even if you do somehow manage to kill it, you can encounter it again as many times as you want in order to complete the friendly monsters sidequest. Just save before you fight it. For the love of god, save.