I think the main problem is not what the goblins actually do but what they get for their mana. Stat-wise they should be on par with their rarity-counterparts. Imo their spells are just too good for what they cost. The solution there is obvious. Either lower their magic/overall spell effectiveness or increase their mana cost. I would say even a small adjustement would suffice as they are not completely broken right now.
The first option would still allow them to capitalize on chaining extra turns easily due to low mana cost abilities while with the second option the extra turn would be a kind of luxury and not a ticket to a potentially very large amount of further bonus turns (depending on the setup).