What is Marvel ?

If he were to ever face an opponent stronger than him, he would get angry that he is losing, then get the strength to defeat them, it's as simple as that.

The concept of his strength being unlimited has been confirmed by three very reliable sources.

The first source was Beyonder, an omnipotent cosmic entity who definitely knows what he's talking about.

The second source was Stranger, an entity that created the living planets and supported Beyonder's claim.

the third source was The Mad Thinker, whose calculations all but confirmed that The Hulk is the strongest superhero in the Marvel universe.