Pretty easy. Assuming you already know the basic flow of the game.
In Campaign, select Rumble in the Jungle. Easy 1 on 1 challenge
You and Garruk have 2 life each.
He only has out a Stomper cub, you have a Runeclaw bear, Barony vampire and a Gloomhunter.
He attacks with Stomper cub. Block that with your Runeclaw bear and your Barony vampire by pressing x to pause the game and select your blockers during his attacking phase. You will take 1 damage and lose those two creatures, but you kill his Stomper cub and that leaves your flying Gloomhunter to hit him for 2 for the win.