Because he is a creation of Galactus, and as such has the power cosmic at his disposal, the same Power Cosmic that Surfer said the Serpent might grow beyond.
but including him is over kill imo. I need to see the limits of the Worthys first...what type of attacks it takes to bypass their durability. From what we have seen so far, it takes much more than high Herald power to stop them but Tyrant is much more than a Herald.
Tickle? Can anyone do anything to Tyrant that won't feel like a gentle spring rain is the more appropriate question. An amped Thanos gave Tyrant far less of a workout than a baseline thanos gave Odin.
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"It is unwise to battle the Power Cosmic." - Silver Surfer
The heralds include Surfer, Terrax, stardust, Morg, Firelord, Red Shift, Nova. This team is enough to take down the worthy. Even Nova would beat Skadi. Kuurth got a beat down by Colossus and was called back. I don't see how guys like morg and surfer wouldn't push him to the limit.
Destroyer, Alpha Ray, and Fallen One aren't even needed.
The Worthy need to prove how strong they are if they are to eclipse every herald's high-end feats.
Btw, Surfer's comment was just a speculation. You know how writers hype the storylines (though it would be sad if it actually turns out to be true. That would be ignorance of the PC's past feats)
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"Farewell, Damos... Ash, Pikachu... And you. All of my beloved." -- Arceus