After completing Fillmore's second Act and squeezing as many rewards from the town's populace as is possible, fly the temple over Bloodpool (the area with the large red lake), save and then send your spirit down to wreak havoc as a warrior once again.
The boss of Bloodpool Act I is the Sphynx, a winged lion-like creature that hops around and spits fireballs. It's relatively easy to kill, since it tries to avoid you rather than pouncing on you. When it hops out of the water on the opposite platform, jump up, then fall back down after the blast has passed. Hop across before it fires again or after it fires, jump to the second platform and strike it. If you can chase it to the bottom platform where it leaps back into the water, you should be able to get two to three swings in while it's diving.