Make fire giants (possibly only in the Valkyrie quest, possibly others) spellcasters who can only cast a fireball spell. Or, to avoid a flavor issue with giants being not very smart to be good at spells, flavor it as something other than a spell (it could also be implemented as a new attack type).

Assuming that shallow water is implemented: all giants should use the shallow water behavior of only wetting their feet even when walking through deep water. Also, fire giants can wade through lava, which acts mostly the same.