Greven and Immolating Souleater on the battlefield.
Greven does not have summoning sickness and cannot be blocked by an opponent.Your life total is at least 17.
Steps
Activate Immolating Souleater eight times by paying 16 life, giving Immolating Souleater +8/+0 until end of turn and causing Greven to become a 21/21 until end of turn due to its first ability.
Deal combat damage to an opponent using Greven.
The opponent loses the game due to being dealt 21 combat damage from your commander.