"Keep your heels down and bend at the hips as your mount takes flight. She'll do the rest."
—Anders, cathar drillmaster

Gryff Rider

Flying
Training (Whenever this creature attacks with another creature with greater power, put a +1/+1 counter on this creature.)

Innistrad: Double Feature

Innistrad: Crimson Vow

A creature's training ability triggers only when both that creature and a creature with greater power are declared as attackers. Increasing a creature's power after attackers are declared won't cause a training ability to trigger.

Once a creature's training ability has triggered, destroying the other attacking creature or reducing its power won't stop the creature with training from getting a +1/+1 counter.