The following page contains the walkthrough for the "The Heart of the Matter" mission in Marvel's Spider-Man. We're approaching the finale of the game - you need to find Mister Negative and face him one last time.
The mission will begin in a heavily-guarded location. First thing you'll need to do is taking out the snipers, then you can dispatch the rest of the enemies. The Inner Demons will throw a lot into this fight to try to stop you. You'll be facing Brutes, the guys with whips and all the other most dangerous enemies.
After a long battle even more enemies will appear - this time it's the guys from Sable. Use the gadgets and web to keep it short. After the fight is finally over, enter the building and chase after Martin Li.
Follow the traces of demonic energy. Drop down and you'll find your adversary. He will teleport into a different dimension; stay on him and he'll be showing you various visions. Eventually, you'll reach a photo of Li's parents, which will take you back to your reality.
It's time to duel Mister Negative himself. The boss will use a sword, and with an exceptional proficiency. Additionally, he'll charge energy and shoot at you. Throw monitors at him and wait for the moment when weakened Li drop the ground. Use that moment to counter him with the triangle.
The battle will follow a certain scheme for a time - there will be some occasional QTEs on the way. Rely on dodging and try to attack when the enemy finishes a combo.
Once again you'll be moved to another dimension. Take out the mobs quickly and get back to Martin. He'll have a similar move set, but the change-into-a-big-demon thing is new - avoid his attacks using jumps and gliding on the web.
The biggest problem may be the large number of specters - try to take them out first using, e.g. gadgets or the costume's special ability. Try to survive this part of the fight by using the web to escape and waiting for the moment when Li is weakened.
After the fight, you'll see a lengthy cut-scene - Doctor Octopus enters the fray and proceeds to wipe the floor with Peter, leaving him in critical condition... After a rest in F.E.A.S.T., our protagonist prepares for the final battle. The reward for completing the mission: 5500 XP