One place to start is looking at procrastination and duskmantle.
-Both yield roughly 50% damage reductions like this shield.
-Both cost 6
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-Both give reduction by "preventing" attacks (either through delay or avoidance)
-Both leave ability intact
This card:
-Yields 50% damage reduction
-Forcibly changes active skill
-Does not actually "prevent" attacks, just reduces damage dealt
-Damage reduction is countered if opponent spends 1
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per attacker per turn
-Leaves attackers protected from being targeted
So the questions I see here are:
1) How much cost-value should be given to auto-skill removal on a shield (This the most important factor I think)
2) How much of a counterbalancing effect does making attackers untargetable add
3) How much counterbalancing does allowing the enemy to negate the effect using
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add
4) How should the fact that no attacks get fully stopped be factored in (if at all)