Can I use Not So Fast to counter Elder Dragon's 'When you play me' ability?

Ruling: Yes, you can use Not So Fast to counter Elder Dragon's "When you play me" triggered ability. The ability chooses enemy units from the Elder Dragon player's perspective, which are friendly units from your perspective, meeting Not So Fast's requirement to counter abilities that choose friendly units or gear.

Sequence:

• Opponent plays Elder Dragon (unit enters board immediately)
• Elder Dragon's "When you play me" triggered ability is placed on the chain, choosing your unit(s) as targets
• Game state becomes CLOSED (chain exists)
• You gain priority and can play Not So Fast as a [REACTION], targeting the triggered ability
• Chain resolves in LIFO order: Not So Fast resolves first, countering Elder Dragon's ability
• The countered ability is removed from the chain and does no damage

Nuances:

• Elder Dragon has two abilities: a passive ability ("Any amount of your damage is enough to kill enemy units") which cannot be countered, and the triggered ability which can be countered
• The term "friendly" is relative to the controller of the card being played - your units are "friendly" from your perspective even though they are "enemy" from your opponent's perspective
• You cannot react to the unit itself being played, only to the triggered ability after it enters the board