Quandrix Command + Crystalline Crawler + Dualcaster Mage + Conclave Mentor

Quandrix Command

Crystalline Crawler

Dualcaster Mage

Conclave Mentor

How it works

1. Cast Quandrix Command by paying its mana cost, choosing its first and third modes.
2. Holding priority, cast Dualcaster Mage by paying its mana cost.
3. When Dualcaster Mage enters, it triggers, copying Quandrix Command, with the copy's first mode targeting Dualcaster Mage and the copy's third mode targeting Crystalline Crawler.
4. Resolve the copy of Quandrix Command, returning Dualcaster Mage from the battlefield to your hand and putting three +1/+1 counters on Crystalline Crawler due to Conclave Mentor.
5. Holding priority, activate Crystalline Crawler three times by removing three +1/+1 counters from it, adding 1.
6. Repeat from step 2.