Counter Strike
Card Text & Abilities
[Reaction] (Play any time, even before spells and abilities resolve.) Choose a unit. The next time that unit would be dealt damage this turn, prevent it. Draw 1.
Official Rules FAQ & Clarifications
1 Official FAQElder Dragon’s passive ability changes the definition for what is considered lethal damage for a unit, as long as that unit has damage marked by Elder Dragon’s controller on it. However, this doesn’t change the actual amount of damage being dealt. Elder Dragon deals 1 damage to up to one unit at each location—if one of those units has a prevent effect applied to it, that prevent effect will replace as much of that damage as it can. In the case of Counter Strike, it will replace all of the damage. If the damage is entirely prevented, it is never dealt per rule 437.2.a., and thus there is no opportunity for the passive ability of Elder Dragon to apply to that unit.