I had the same problem. What I did was I got a whole bunch of rupees(so you can get the heat resistant armor), and just run as fast as I can to goron city, healing myself when my health got too low.
I had decent amount of hearts, used heat resistant potions was still dying and ran as quick as I could and kept healing. You need enough rupees to buy a piece of heat armour in Goron City.
When I got the heat resistant armor set I went to the great fairy and got all pieces to two stars and that makes you fire proof. It helps in the death mountain area. Grab those fireproof lizards. The great fairy needs those especially for upgrading.