Pentavus + Dross Scorpion + Firemind Vessel

Steps

1. Activate Firemind Vessel by tapping it, adding {2}
2. Activate Pentavus's first ability by paying {1} and removing a +1/+1 counter from it, creating a 1/1 Pentavite creature token.
3. Activate Pentavus's second ability by paying {1} and sacrificing the Pentavite.
4. When the Pentavite dies, Dross Scorpion triggers, untapping Firemind Vessel.
5. Resolve the Pentavus ability, putting a +1/+1 counter on it.
6. Repeat.