Does Not So Fast counter the full trigger of Elder Dragon's "When you play me" effect, or would Not So Fast only counter the trigger on one unit?
Yes, Not So Fast counters the entire "When you play me" triggered ability of Elder Dragon, protecting all units targeted by that ability.
According to RiftJudge FAQ #10330, when you use Not So Fast to counter a spell or ability, you are countering the entire effect as a single chain item, regardless of how many units it was intended to affect. Not So Fast does not partially resolve; it cancels the entire ability, so none of its instructions (the damage to your units) will execute.
Sequence:
- Opponent plays Elder Dragon: The unit enters the board.
- Triggered ability: Elder Dragon’s "When you play me" ability is placed on the chain, choosing up to three of your units as targets.
- Game State: The state becomes CLOSED.
- Reaction: You play Not So Fast as a [REACTION], targeting the Elder Dragon's triggered ability.
- Resolution: The chain resolves in Last-In, First-Out (LIFO) order.
- Not So Fast resolves first, countering the Elder Dragon’s triggered ability.
- The Elder Dragon’s ability is removed from the chain and does no damage to any of your units.
According to Rule 425.1.a, a card or ability that is Countered does nothing and is cleared from the chain.