Valkyrie's Call + Amoeboid Changeling + Spawning Pit + Zealous Conscripts

Valkyrie's Call

Amoeboid Changeling

Spawning Pit

Zealous Conscripts

How it works

1. Activate Amoeboid Changeling's second ability by tapping it, removing all creature types from Zealous Conscripts.
2. Activate Spawning Pit by sacrificing Zealous Conscripts.
3. When Zealous Conscripts dies, Valkyrie's Call triggers, returning Zealous Conscripts from your graveyard to the battlefield as an Angel with flying and a +1/+1 counter on it.
4. When Zealous Conscripts enters the battlefield, it triggers, untapping Amoeboid Changeling.
5. Resolve the Spawning Pit ability, putting a charge counter on it.
6. Repeat.