Well @Urmi that's pretty much the issue. If a villain is unbeatable, or worse not defeated by the player, they're a bad villain.
A villain can serve two major purposes. One, they can make you question your stance on the world and how you see it. If they make you think, "Oh, maybe they have a point..." That makes a good villain. One who challenges your morals. Seppy definitely was not this.
Two, they can serve as a hurdle to be overcome. A mountain to be climbed. A test to succeed at, and show just how far you've come. It was one thing to have Seppy be powerful at the beginning, when we, the player, the main character, were not, but to have him still be that same obnoxious power fantasy at the end? That's where Seppy failed at the second criteria.
As you said, it was pretty much The Hulk vs. a toddler, but unlike Bruce Banner, we didn't see how Seppy got his power, we didn't see how he got so strong, unlike The Baron even. With Seppy is was just an annoying power fantasy that we had to suffer through, a creator's pet who is "totally teh bestest bad guy evur!"
He wasn't representative of anything, he wasn't Pyramid Head, he wasn't an analogy to the inexorable march of time, and by extension decay, no.
He was just a guy in a spiky suit of armor with a talking sword of EVIL! who was trying to take over the world. Just a Saturday morning cartoon villain on steroids.
@Luis Gotta get dem clicks boi