Eating the corpse
Eating the corpse of a Fire giant gives you a 30% chance of gaining the fire resistance intrinsic, which can be considered critical for Gehennom. You can also increase your strength when you eat a giant corpse, by the same amount as when you eat a non-cursed tin of spinach.
Valkyries will find many fire giants on their quest, and, along with the fire ants also found there, they give a very good opportunity for valkyries to get fire resistance, even before they reach level 14. However, this can be risky without reflection, due to the giants' tendency to use attack wands.