to be honest i dont know. I really dont understand how a superpowered alien beat the embodiment of entropy. but then again SS beat tenebrous and aegis using Big Bang energies. Even though SS is a lot more powerful than Supes neither one of them should be beating abstracts.
No he didnt, at least not on his own accord. He had Agies to help. Granted it was stated that both Agies and Tenebrous were on the same level as Galactus, their feat includes fighting and beating Galactus, then dying at the hands of silver surfer who channeled the energies of the church. Not really a very impressive streak i would say.
Then again, Imperix himself doesnt have much a feat to put it out there, he killed doomsday with a blast, but most of the fighting was done through Imperix Probe rather than Imperix Prime.