In particular, indirect elemental spells go against half impact. Vehicles ignore damage that doesn't do base higher than their armor (because that would do stun, and they ignore stun), so indirect elemental spells are one of the best ways to hurt them.
Let me especially recomend Screech/Soundwave. Sonic damage's special effect is that it ignores
all normal armor, while sound dampening effects, like the Hush spell, count as armor equal to their rating, IIRC.