One place to start is looking at procrastination and duskmantle.
-Both yield roughly 50% damage reductions like this shield.
-Both cost 6
-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
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
add
4) How should the fact that no attacks get fully stopped be factored in (if at all)