Yes and No. I know why you would ask this question, but the answer isn´t a simple one. First of all, ill start why they are effective. Fire Giants have no defense against magic. This means that even in full armor, you are going to hit them pretty easily.  There are two no´s. First no, is the fact that there are no good spells to kill them. They take up quite a lot of space, so you can not use your ancient magicks to their fullest potential. The other spells on the normal spellbook are really slow to kill them.  Then this is probably the no you are looking for, elements have no effect on monsters in RuneScape. So a water spell will work just as good or as bad as a fire spell or wind spell would on fire giants.