Cast Trusty Boomerang by paying {1}.
Sokka's second ability and each Ally you control's prowess ability trigger.
Resolve all prowess triggers, giving each Ally you control +1/+1 until end of turn.
Resolve the Sokka trigger, creating a 1/1 Ally creature token.
When the Ally enters, Mana Echoes triggers, adding {C} for each Ally you control.
Activate Trusty Boomerang's equip ability by paying {1}, attaching it to the Ally token.
Activate the Ally token by paying {1} and tapping it, tapping a creature a returning Trusty Boomerang from the battlefield to your hand.
Repeat.

Infinite ETB

Infinite colorless mana

Infinite storm count

Infinite tapped creature tokens

Infinitely large Allies you control until end of turn

Sokka, Tenacious Tactician

Trusty Boomerang

Mana Echoes

Frostcliff Siege

Cast Trusty Boomerang by paying {1}.
Kykar triggers, creating a 1/1 Spirit creature token.
When the Spirit enters, Mana Echoes triggers, adding {C} for each Spirit you control.
Activate Trusty Boomerang's equip ability by paying {1}, attaching it to the Spirit token.
Activate the Spirit token by paying {1} and tapping it, tapping a creature a returning Trusty Boomerang from the battlefield to your hand.
Repeat.

Infinite LTB

Infinite sacrifice triggers

Infinite death triggers

Infinite red mana

Kykar, Wind's Fury

Cast Trusty Boomerang by paying {1}.
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, Wartime Protestors and Mana Echoes trigger.
Resolve the Mana Echoes trigger, adding {C} for each Ally you control.
Resolve the Sokka trigger, attaching Trusty Boomerang to the Ally token.
Resolve the Wartime Protestors trigger, putting a +1/+1 counter on the Ally token and giving it haste until end of turn.
Activate the Ally by paying {1} and tapping it, tapping a creature and returning Trusty Boomerang from the battlefield to your hand.
Repeat.

Wartime Protestors

Sokka and Suki

Cast Trusty Boomerang by paying {1}.
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} for each Ally you control.
Resolve the Sokka trigger, attaching Trusty Boomerang to the Ally token.
Activate the Ally by paying {1} and tapping it, tapping a creature and returning Trusty Boomerang from the battlefield to your hand.
Repeat.

Fervor

Activate any number of Birds, Frogs, Otters and/or Rats by tapping them, adding at least {3}.
Cast Trusty Boomerang by paying {1}.
Valley Floodcaller triggers, giving Birds, Frogs, Otters and Rats you control +1/+1 until end of turn and untapping them.
Activate Trusty Boomerang's equip ability by paying {1}, attaching it to a Bird, Frog, Otter or Rat.
Activate the creature by paying {1} and tapping it, returning Trusty Boomerang from the battlefield to your hand.
Repeat.

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