Mechtitan Core + Prototype Portal + Myr Battlesphere + Dross Scorpion + Ashnod's Altar

Mechtitan Core

Prototype Portal

Myr Battlesphere

Dross Scorpion

Ashnod's Altar

How it works

1. Activate Prototype Portal by paying 2 and tapping it, creating a token copy of Mechtitan Core.
2. Activate the Mechtitan Core token by paying 5 and exiling it, Myr Battlesphere, and any three other nontoken artifact creatures and/or nontoken Vehicles, creating a 10/10 Mechtitan creature token.
3. Activate Ashnod's Altar by sacrificing the Mechtitan token, adding CC.
4. The Mechtitan token dies, triggering Dross Scorpion and Mechtitan Core.
5. Resolve the Dross Scorpion trigger, untapping Prototype Portal.
6. Resolve the Mechtitan Core token trigger, returning all other cards exiled by it to the battlefield tapped.
7. Myr Battlesphere enters the battlefield, creating four 1/1 Myr artifact creature tokens.
8. Activate Ashnod's Altar three times by sacrificing three Myr tokens, adding CCCCCC.
9. Dross Scorpion triggers three times, allowing you to untap up to three artifacts you control.
10. Repeat.