Devoted Druid, and Captain Rex Nebula

Prerequisites

• Devoted Druid on the battlefield
• Captain Rex Nebula on the battlefield

Steps

1. At the beginning of combat, Captain Rex Nebula triggers, making Devoted Druid a Vehicle until end of turn
2. Activate Devoted Druid's first activated ability by tapping it, adding
3. Activate Devoted Druid's second activated ability by putting a -1/-1 counter on it, untapping it
4. Repeat from step 2

Results

• Infinite green mana