Valkyrie's Call + Amoeboid Changeling + Spawning Pit + Bounding Krasis

Valkyrie's Call

Amoeboid Changeling

Spawning Pit

Bounding Krasis

How it works

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