Valkyrie's Call + Amoeboid Changeling + Spawning Pit + Sparring Mummy

Valkyrie's Call

Amoeboid Changeling

Spawning Pit

Sparring Mummy

How it works

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