Quandrix Command + Crystalline Crawler + Dualcaster Mage + Benevolent Hydra

Quandrix Command

Crystalline Crawler

Dualcaster Mage

Benevolent Hydra

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 Benevolent Hydra.
5. Holding priority, activate Crystalline Crawler three times by removing three +1/+1 counters from it, adding 1.
6. Repeat from step 2.

This ingenious loop harnesses Quandrix Command to perpetually recycle Dualcaster Mage back to your hand while steadily swelling your board state. Every time Dualcaster Mage re-enters, it copies the Command to bounce itself and empower Crystalline Crawler, whose counters are supercharged by Benevolent Hydra's replacement effect. You then cash in those extra counters from Crystalline Crawler for the exact mana needed to recast Dualcaster Mage, sustaining an endless chain of entering and leaving the battlefield.