This page lists resistances for various enemy types in Dragon Age II. Note that elemental immunity is only applied on the Nightmare difficulty; on other modes, it is set to normal. In general, frost and electricity are the most useful elemental types for runes as the act 2 and 3 end game enemies are weak to frost. Dragons are also weak against frost, and most undead are weak against electricity. Most non templar humans fought in Kirkwall will be weak vs Fire as well.