Hudson held on for absolutely zero time on his own. The alien was taking him into the vent. It was only because Hicks was there to hold onto Hudson. That's two marines to one alien. Aliens have the capacity to bash through metal doors, lift a human being effortlessly into the sealing.
You keep saying Parker wasn't "trained" there is no anti-xenomorph hand-to-hand training in the colonial marine regiment. Parker was a tough cookie, and he was reduced to nothing in seconds. The alien has the advantage in physical strength, always has.
Do you think a human, even a trained marine, could successfully grapple with a lion without at the very least, being seriously hurt? Probaly not. Well, an Alien is even worse.