Tephraderm + Hive Mind + Star of Extinction + City on Fire

Tephraderm

Hive Mind

Star of Extinction

City on Fire

How it works

1. Give Tephraderm indestructible, if needed.
2. Cast Star of Extinction by paying 5.
3. Hive Mind triggers, causing each opponent to copy Star of Extinction.
4. Resolve an opponent's Star of Extinction, destroying a land and dealing 20 damage to each creature and planeswalker.
5. Tephraderm's second ability triggers, dealing 60 damage to the opponent who controlled the Star of Extinction copy due to City on Fire.
6. Repeat from step 4 for each remaining Star of Extinction until each opponent loses the game.