This might be due to mana potions, the somewhat insufficiently tested gimmick of this campaign.
Just had this problem again in Guild Wars. Enemy with 70 Attack instakilled every member of my team with much more armor/health, each with a single normal skull match. Lost two of todays matches because of that 
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If they have something that makes skulls and it makes 2 or 3 separate skull matches that counts as separate damaging attacks.
Gnolls and guild guardians do double damage against certain colors. Webspiner does triple skull dmg against webbed enemies, the catch that it webs the enemy on skull dmg and does it before the actual skull dmg, so actually does triple dmg all the time.
It was a single 3 skull match from a normal board each time. No alignments on my side and no skills on the enemy side. If that would have been the case I wouldn’t have written here.
And after reducing the speed the same attack had less damage in the third game, which I won easy, but 1x is slow as hell. Hard to play after 4x.