Can't decide on a second mastery (Vit/Cold build)

I’m not even sure what I’m going for here, but I started a new character as a strict co-op with Necromancer as my main. I was thinking of a Vit/Cold build, but I can’t decide on a second mastery. The person I’m playing with is going for a ranged firestrike build, so I’m trying to offset the fire damage with the opposite so we won’t run into any enemies that are able to resist both of us easily.