When it comes to the most powerful characters in the Marvel Cinematic Universe, the answer is often split between those who have immense strength or considerable energy output. For example, Thanos is incredibly powerful due to his strength and ability to withstand the Infinity Gauntlet. At the same time, Captain Marvel is nearly indestructible and absorbs immense amounts of energy. As a result, even characters like the Incredible Hulk still have to fight for the top spot. But, unlike Thanos and Captain Marvel, Hulk has already demonstrated his vulnerabilities -- and how he handles them.
Since his cinematic debut in 2008, Hulk has shown he can withstand bullets, missiles and sound-based weapons. Due to the gamma radiation that birthed him, his skin is nearly indestructible and takes massive force to break. Throughout Hulk's many on-screen fights, he has only bled twice, and both circumstances prove it takes a unique set of variables to accomplish that.
Abomination's Spikes Pierced Hulk's Skin - But How?
In The Incredible Hulk, Green Goliath battles his powerful nemesis, The Abomination. Unlike what has been shown in the comics, this character's origin involves more than exposure to gamma radiation. Because of Emil Blonsky's desire to keep fighting even past his prime, General Ross signs him up to undergo a Super Soldier program, similar to Steve Rogers. While it works, Blonsky's body mutates as his desire for more power grows. This causes him to go rogue and strongarm Samuel Sterns, aka The Leader, into giving him a dose of Bruce Banner's irradiated blood and exposure to gamma rays. Thus, The Abomination is born.
The first phase of his mutation amplifies areas where he was injected with the Super Soldier Serum, like his spine. This causes his bones to protrude from his back and elbows. It also changes his form, so he resembles Hulk and is even taller and stronger. As a result of his mixture of serums, Blonsky is also stronger than Hulk. This allows him to overpower the hero for a short time. With Hulk pinned, Blonsky forces him to watch Betty Ross die in a fiery explosion, stabbing his elbow into Hulk's chest. This marks the first time that Hulk bleeds in the MCU. The scene also shows off how strong Blonsky is, and his later appearance in Shang-Chi and the Legend of the Ten Rings indicates that his bone protrusions are a stage in his continuously evolving physiology, even if his motivations are more benevolent.
Why Could Fenris Penetrate Hulk's Skin?
The second time that Hulk bleeds is in Thor: Ragnarok. By the time he is reintroduced in the film, Hulk has gotten stronger and more intelligent after living on the gladiatorial planet of Sakaar for two years. Due to his increasing strength and skill, his skin may have gotten stronger as well. But this is put to the test when he faces off against Hela's giant resurrected wolf, Fenris, on Asgard. During the Battle of the Rainbow Bridge, the two take their fight to the water, and Fenris rag dolls Hulk before biting into his leg, making him bleed.
Even though Hulk has gotten stronger and faced deadlier foes since The Abomination, it makes sense that Fenris could hurt him because the wolf is of Asgardian descent. It's been shown in the past that people from Asgard can hurt Hulk, so it makes sense that a wolf of that size could easily injure him. There's also the added boost that the Eternal Flame brought Fenris back to life. Because magic is coursing through the wolf's veins, this further amplifies his power and puts Hulk at a slight disadvantage. But even though his weaknesses have been revealed, what makes Hulk so impressive is his ability to take these hits and keep fighting, angrier and more powerful than before. In fact, even as Smart Hulk, it's likely that this hero could still put up a good fight before being injured enough to bleed.