rip out his heart, its very simple. He has a healing factor, which means he can only heal broken skin and bones, he does not have a regenerative factor such as Deadpool were he can regenerate lost organs, etc.
Hulk can regrow any organ, bone, or tissue, (even his brain,) in a matter of moments. His brain isn't what causes him to regenerate; it's his DNA. Hulk's DNA instructs his cells to replicate at an unimaginable rate. The rate at which his cells multiply increases with the amount of physical or emotional stress he experiences. Destroying every DNA molecule in Hulk's body may be the only physical means of killing him. Otherwise, he'll keep coming back.
You can't kill Bruce before he turns into the Hulk. Any injury he sustains could result in immediate transformation, and the wound would heal. Bruce has tried to kill himself in the past. Obviously, he hasn't been sucessful. The Hulk knows that if Bruce dies, he dies as well.
However, after an extremely long period of being the Hulk, Bruce has to rest. That is his point of vulnerability.
Hulk has been hit by beams meant to cut adamantium, taking only minor damage that healed almost instantly. Hulk would heal just as fast as the blade cut him, and it probably would never breech his skin.
Wrong, Darkcrawler. If Hulk got hit by a beam that cuts adamantium, that moves at lightspeed without it penetrating him, why would Flash, who moves at lightspeed, be able to cut him with an adamantium blade?
Hulk's skin doesn't burn easily.
Hulk has demonstrated the ability to regenerate from being reduced to gamma irradiated cells. Cutting his head off isn't going to work.