1) This cannot stack with regular damage. The opponent must be fully healed for this to damage them. This is a vital distinction to consider.
2) This costs more than Poison. (not much more but more 1
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~= 3
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, 5
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~= 2
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)
3) People can defend against this with PC, Denial and Max Hp gain. People can defend against Poison with Purify, Healing and Denial.
4) Every element having access is not a balance issue unless there is a problematic synergy. It may be a thematic issue but that is the nature of shards.
I personally think these 4 factors balance.