To kill blackheart you will need the white hot shotgun and the plasma shotgun combo. you have to dodge his attacks, and when he slams one of his hands down, shoot it with the white hot shotgun. when his head is on the ground, attack him with the plasma shotgun combo. Then, he will attack again, so stay in the center as much as possible because he will eventually destroy both chunks of building on the left and right sides. wait until he slams one hand down, then repeat the process.