Thranduil, the Elvenking, Devoted Druid, and Timberwatch Elf

Prerequisites

• Thranduil, the Elvenking on the battlefield
• Devoted Druid in the graveyard
• Timberwatch Elf in the graveyard

Steps

1. Activate Thranduil using Timberwatch Elf's ability by tapping it, giving Thranduil +1/+1 until end of turn for each Elf you control
2. Activate Thranduil using Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it
3. Repeat

Results

• Infinite green mana
• Infinitely large creatures you control until end of turn