Trusty Boomerang + Mana Echoes + Dragonlord Kolaghan + Sokka and Suki

Trusty Boomerang

Mana Echoes

Dragonlord Kolaghan

Sokka and Suki

How it works

1. Cast Trusty Boomerang by paying 1.
2. When Trusty Boomerang enters, Sokka and Suki's second ability triggers, creating a 1/1 Ally creature token.
3. When the Ally enters, Sokka's first ability and Mana Echoes trigger.
4. Resolve the Mana Echoes trigger, adding C for each Ally you control.
5. Resolve the Sokka trigger, attaching Trusty Boomerang to the Ally token.
6. Activate the Ally by paying 1 and tapping it, tapping a creature and returning Trusty Boomerang from the battlefield to your hand.
7. Repeat.