Trusty Boomerang | Mana Echoes | The Fire Crystal (and one other card)

Cards

Cast
Trusty Boomerang by paying ({1} magic symbol) .
When
Trusty Boomerang enters,
Sokka and Suki's second ability triggers, creating a 1/1 Ally creature token.
When the Ally enters, Sokka's first ability and
Mana Echoes trigger.
Resolve the
Mana Echoes trigger, adding ({C} magic symbol) for each Ally you control.
Resolve the Sokka trigger, attaching
Trusty Boomerang to the Ally token.
Activate the Ally by paying ({1} magic symbol) and tapping it, tapping a creature and returning
Trusty Boomerang from the battlefield to your hand.
Repeat.

This combo has been estimated to be Exhibition (Bracket 1+).