[thejeff]

I still think the best counter to Neifel Giants is to one-shot them. Even with a good mr, at least the ones you get die. (Or get enslaved, even better.)

With their regen you're going to have to hit them at least 2-3 times in quick succession to kill with magma bolts, or lighting, or fire arrows.
Maybe falling fires or better yet, magma eruption. Decent area, good damage and leaves a heat aura behind.