Darkling Shamans can stack Shrink multiple times on one unit - causing 0 physical attack after second cast (and possibly extra dodge, I'm not sure).
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I had a battle where a Darkling Shaman cast Shrink 3 times on the one unit. After the second cast though the attack of the affected unit was reduced to 0. (I presume it only affects physical damage, but Shrink should not be able to be stacked on any unit from any source.)