Scrap Trawler + Myr Retriever + Phyrexian Altar + Temur Battlecrier

Scrap Trawler

Myr Retriever

Phyrexian Altar

Temur Battlecrier

How it works

1. Activate Phyrexian Altar by sacrificing Myr Retriever, adding one mana of any color.
2. When Myr Retriever dies, it triggers, returning Scrap Trawler from your graveyard to your hand.
3. Cast Scrap Trawler by paying its mana cost, which is reduced by Temur Battlecrier.
4. Activate Phyrexian Altar by sacrificing Scrap Trawler, adding one mana of any color.
5. When Scrap Trawler dies, it triggers, returning Myr Retriever from your graveyard to your hand.
6. Cast Myr Retriever by paying its mana cost, which is reduced by Temur Battlecrier.
7. Repeat.