Valkyrie's Call + Amoeboid Changeling + Spawning Pit + Breaching Hippocamp

Valkyrie's Call

Amoeboid Changeling

Spawning Pit

Breaching Hippocamp

How it works

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