okay. here's what to do. walk in a big circle. till you find the right spot (a side you favor). then just stop there and let him spray out fire. then roll to his sid and hit his shell with the Cane of pacci. the shell will flip upside down and knock him silly. walk on his neck and smack his back with your sword repeatedly for about 3 seconds then run back to land as he'll sink into to lava. then you repeat the process until he's defeated. I might warn you though that the level of lava on the floor changes each round. if you're down on health, slash the fire with your sword. there might be a heart or two.
actually you can, you just gotta move right before he sprays the fire or you'll be barraged. your sheild will keep you from getting singed. and, the flames are cuttable like grass and hold item in them sometimes. mostly hearts. just stick to the above and keep to the same pattern.i have five hearts and i know how to beat him but hes sparys fire everywhere so i cant roll around to dodge his attacks and hit the shell on his back.