Retreat to Coralhelm + Llanowar Scout + Arid Archway

Retreat to Coralhelm

Llanowar Scout

Arid Archway

How it works

1. Activate Llanowar Scout by tapping it, putting Arid Archway from your hand onto the battlefield.
2. When Arid Archway enters the battlefield, it and Retreat to Coralhelm trigger.
3. Resolve the Arid Archway trigger, returning itself from the battlefield to your hand.
4. Resolve the Retreat to Coralhelm trigger, untapping Llanowar Scout.
5. Repeat.