The Pharaoh boss is rather easy once you know his patterns. The best character for this is Lawrence, since he has high range and power (although he's a bit slow). What you want to do is stay in the middle of the screen during the times where he is up and about. Use the markings on the floor as a "guide", so to speak, to indicate where to stand. Never get too close to him, and don't ever try to attack him in the air. If you are standing right outside of the range of his eye lasers, you should be able to get in a couple of hits when he walks towards you. Just don't move in too close or he'll whack you with his stick. Once he crumbles, swing away at the ball until you see the stick go flying; at that point, stop attacking and be prepared to jump over it when it returns. This is a rather boring, tedious battle (the worst in the game, imo, even worse than the stupid giant flower early in the game).
DON'T use Rumi for this or you will be there all friggin' day. Her knives are weak and hit only 50% of the time. Don't use Sadler either, he doesn't have enough range or power. You could do a little bit of extra damage when he crumbles by using Kindi (about the only time in the entire game where this otherwise useless doofus is actually useful) but it's not really enough, imo, to justify switching characters.
Joe: the usage of multiple tonics can also fully restore your life or hyperboost your stats temporarily. It's a real gamble, but one not really worth taking.