Isn't Fernus basically MMH with no limits? He still loses than. J'on had to use a once in a life time ability to trick Despero into losing. Which I think I can only think of one instance where anyone didn't resort to cheating to beat Despero. This guy is a nightmare to try and solo.
he is much more powerful, Martian manhunter can get the joker sane for a while, fernus made the whole asylum sane
Fernus isn't more powerful than Martian Manhunter, he' an aspect of Martian Manhunter rather than a completely different entity
Actually they closed the story off by clearing J'onn of any wrong doing as Fernuswas by the law considered a separate biological entity.
That said, Fernus was just a no-limits Martian Manhunter. And while Despero may have sheer force on his side regarding TP, I don't think he has Fernus' finess. (By that I mean that Despero's usual trick with TP is to mindcontrol his opponent, Fernus however could paralyze individual muscles)