The version of Doc Ock you see in "Spider-Man: No Way Home" is ultimately pretty easy to defeat once Spider-Man (Tom Holland) figures out a way to override the tentacles with his Stark nanotech. But, before that hack comes in play, the villain is a borderline invincible threat who handily beats the hero. So, how did he become so dangerous in the first place?
In "Spider-Man 2," Dr. Octavius is a perfectly pleasant nuclear scientist before things go awry. He develops the artificial intelligence tentacle harness in order to help him to accurately and safely manipulate the dangerous materials he needs in his fusion power project, which explains their sturdiness and dexterity. Unfortunately, he needs to use an inhibitor chip to keep this sophisticated piece of tech from tampering with his nervous system. When said chip gets wrecked in an accident and the harness becomes permanently fused with him, the AI of the tentacles effectively takes him over and overwrites his personality, communicating with Octavius via disembodied voices in his head. The tentacles can operate even when Doc Ock is unconscious or focused on something completely different, because they quite literally have a mind of their own, and can use their own built-in cameras to observe the environment. As such, it's virtually impossible to catch the villain off his guard, even if he has to fight multiple people at once.
As such, the grouchy, villainous version of Doc Ock you see is ultimately just a fleshy, tortured mouthpiece for the sentient AI tentacle harness system that acts as an independent extension of his body, both protecting and controlling him. No wonder the guy is so eager to help the Spider-Men after his inhibitor chip is finally restored.
