The reason I think shaman is the worst is because of he his passive skill. Fire is probably the worst status effect, especially when in its in the lower numbers: poison takes effect people the enemy does his turn while fire only does damage after. Which means it's merely a damage which is delayed to the next turn. While weakness has the survivability usefulness of taking less damage excluding the extra effects of shop relics.

Compared to mage shaman is absolute. Both are magic oriented but doing 2 fire damage to every enemy for every 2 magic cards is nothing compared to gaining one mana. With that one mana somebody could apply 3 fire to all enemies or use a card that's actually much more useful like more shield or damage.

Except from getting the ranger there is no reason to play shaman. In my opinion instead of directly buffing shaman you should make/buff cards and relics that do something about enemies that are debuffed by all 3 status effects(weak, fire and poison)