To defeat Firash hit him 6 times with water. After each hit he dives back down into the lava- use your heads-up display to figure out where he will pop up (look for the big red dot).
When Firash stops moving for a moment target his head but be sure you are lose enough to be in range. To conserve fuel you can rest on the stone structure in the middle of the area. There are also Hearts and Catana located around the perimeter if needed.