So this is the Magmadar boss fight in Molten Core. Before you criticize it too much, realize its the first time I've ever been there. For Warlocks the best way to do this is to use drain life on the target, tap life for mana and heal between fears - especially when the rest of your guild mates are dead. I used curse of shadow, but I'm not sure if that helped any. I did this without any fire resistance gear, but you don't really need it. Just bring some potions and use fire shield.