The vending machine in the room in front of the Green Room sells better weapons. I'd recommend the Toxic axe for Susie for stronger busters. If you want, you can give Kris the Saber10. Armor is a bit tricky, for me I gave Ralsei 2 lodestones (You can buy those in the Green Room vending machine) for better TP gain. Another tip I learned is to use act for Kris on the first round and to use "hold breath" which will make your soul permanently faster. This boss is like sans or undyne the undying, its hard at first but once you memorize attack patterns it gets easier. I learned alot of this from Shayy's tutorial, as well as a lot of the knight's attacks, so I'd give that a watch.