Nowhere did he say it had to be in GUI, since this is the spells and systems section, both are allowed. And with AoS that problem would only occure if the tower is all alone which points to a map flaw. But anyway, why make it make the target unkaillable when it would be fine and more strategic if it did not change the damage the target receives at all. It would force players to think before they use, going to a nearly dead tower would be a waste. But it is your map so you chose but I recommend you use handle vars (or systems that are simlar), they are the easiest way.