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If it wasn't because Blake was constantly using Mana to maintain his huge Familiar Army, he would have let his Vampire Spirits go completely all-out, making it so they could use their Powers much easily. However, at this point, Woo and Fang were at their limits.
Any other foe wouldn't be so hard, but this huge gorilla was a servant of Satan, the Archdemon of Wrath. All of his demons were known for having tremendously strong physical bodies, while lacking much in the magic department, aside from a few exceptions.
Often times, they were either beast-like or reptile-like in appearance, and with the small shards the Archdemon gave to his strongest subjects, they could transform into second, even mightier and stronger forms.
With such strong muscles and scale armors covering Kong's body, his pressure points were always well protected, and his muscles, harder than most metals, could simply not be destroyed by just kicks and fists alone anymore.
Check "The Protagnist's Sister Is Actually the Strongest"!