18F is consistently dangerous, unfortunately, if you're at a low-ish level. Pretty much every phase will have some attack that can screw you over.
But yeah, the first phase is easily the most dangerous, because it does the two most dangerous moves (SoHD and the physical attack one) most often. The most important advice I can offer is to focus purely on offense and charge through most of the forms as fast as you can, but understand how his phase changing works, and allow him to survive in the fire phase. It's by far the least dangerous, except if you've got a fire-weak character (Which frankly you shouldn't), and you can safely use that phase to recharge your defenses, heal, and even spend a bit of time focusing to recover SP. Whenever it comes up, use it as a break period instead of focusing on offense, and don't burn that phase away until you're ready to charge through the final form.
That's how I beat it at <100 Highest Level, so yeah.
