Greven does not have summoning sickness and cannot be blocked by an opponent.Your life total is at least 17.
Your deck size is at least eight.
Steps
Activate Wall of Blood seven times by paying 7 life, giving Wall of Blood +7/+7 until end of turn.
Declare Greven as an attacker.
Greven triggers, causing you to sacrifice Wall of Blood, draw seven cards and lose 9 life, 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.