Ben transforms into Echo Echo, multiplies into 20, all 20 Echo echos transform back into human, Ben is now duped into 20 so its 1 ben for each character to fight. Each ben transforms into different aliens in order to fight the other characters.

For Omni Man and Homelander I guess it would be fully blossomed Swampfire. Fully blossomed Swampfire can regenerate, use sleeping spores, and can generate vines

For Dr. Manhattan. No Ben loses to him without Alien X.

For Goku its Ghostfreak. If he uses Ghostfreak he can posses Goku to forcefully snap his neck and scan him to add to his arsenal.

For Superman it could be Gravattack. Gravattack can use his weakness of an uncontrollable black hole to beat him.

For Vilgax it could be Clockwork. Clockwork can use time powers to beat him.

For All Might its also Fully blossomed Swampfire

For Escanor its probably NRG I don't know much about Escanor let alone 7 deadly sins so its my best guess. I've seen some images of Escanor burning and looks super powerful and that his peak is at noon so I think NRG will break out of its containment thing and absorb as much power as he could

For Naruto the alien called The Worst. The Worst's alien species are indestructible he can use it to tire him out with the downside that he can still feel the pain.