Complex sentence: As a human, Dean is the bearer of the Mark of Cain, this grants him extraordinary strength, allowing him to fight on the level of a supercharged angel like Metatron.
Split into simpler sentences: As a human, Dean is the bearer of the Mark of Cain. Granting him extraordinary strength that greatly exceeds that of a human, the Mark of Cain allowed Dean to go toe to toe with a supercharged angel like Metatron.