No he can't. Not in Spawn's reality. Cogliostro told Angela that no one can destroy him. You can contain him, but you cannot destroy him. In much earlier times, it took the combined might of Heaven and Hell to imprison him in a null void. If they could kill him, they would have the first time. Even now a weakened Urizen is in a coma under the Earth, thanks to GreenWorld
Yeah, I would say Urizen wins too. I didn't ask if Onslaught could destroy him, but if maybe there was a chance Onslaught could entrap him, the way he trapped Juggarnaut in the Cyttorak Gem, although not with the same ease.
Urizen cannot be killed by anyone, but he could be defeated, and if Spawn, Angela, and the Greenworld could imprison Urizen, Onslaught must have a chance SOMEWHERE.....
but who knows, Urizen could probably smash Onslaught the minute he sees him, Urizen is like.....400 feet tall, and keeps growing the more soul energy he consumes.