Retreat to Coralhelm + Golgari Rot Farm + Llanowar Scout

Steps

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