Maester Seymour + Crystalline Crawler + Hellkite Charger

Maester Seymour

Crystalline Crawler

Hellkite Charger

How it works

1. At the beginning of your combat phase, Maester Seymour triggers, putting at least seven +1/+1 counters on Crystalline Crawler.
2. Declare Hellkite Charger as an attacker, triggering its ability.
3. Holding priority, activate Crystalline Crawler's first ability seven times by removing seven +1/+1 counters from it, adding 5.
4. Resolve the Hellkite Charger trigger, causing you to pay 5 to untap all attacking creatures you control and to get an additional combat phase after this one.
5. Repeat.