Aethersnipe + Gwenna, Eyes of Gaea + Thryx, the Sudden Storm

Steps

1. Activate Gwenna by tapping it, adding {U}{U} that can only be spent to cast creature spells or activate abilities of creatures.
2. Cast Aethersnipe for its evoke cost by paying {U}{U}.
3. Gwenna triggers, putting a +1/+1 counter on it and untapping it.
4. When Aethersnipe enters the battlefield, its first ability and evoke ability trigger.
5. Resolve the first Aethersnipe trigger, returning it from the battlefield to your hand.
6. Resolve the second Aethersnipe trigger, failing to sacrifice it as it is no longer on the battlefield.
7. Repeat.