Make sure to have full health, loaded weapons, and a Molotov cocktail before riding the elevator to the crematorium. Get some cover and kill Bruno's henchmen first -- they will come after you anyway. Then, throw the Molotov cocktail so it lands near enough to burn Bruno, but not kill him. He will scream and drop his weapon. You will not be able to grab him while he burns, but it will not affect your health if you try it. When the flames stop, grab him and march him to the oven. When his back is against the oven, push him back so his head and shoulders are inside it. Then, pretend like you are really trying to shove him in there with your controller.