What is this goblin spell that’s been wrecking me?

By your description this seems to be Consuming Inferno, but it could also be Wall of Fire. If it's the first, then try to break it with an ability like Shield Slam, if it's the latter, then just don't attack with your weapon while the effect is active and you should be fine. Anyway, getting some fire protection enchantments on your gear will also help to reduce the damage.

As seen on this Reddit post, it seems to be a type of Fire Spell that is easily avoidable by backing away and letting the effect end. Just keep your distance whenever they ready up the spell, run in during their down time and hit them, rinse and repeat until the Shamans are dead. EDIT: As mentioned in the post also, don't try to attack while the spell is active. It seems to hurt you.