I always assumed it was easier because Traven's soul was continuing to help you.
Traven's soul in the colossal gem literally meant the Enthrall spell which is Mannimarco's trademark has absolutely zero effect on the player character.
That's usually the first and only spell Mannimarco has to cast in combat. It's likely his other combat and magical skills atrophied over the centuries because that's all he has to do to defeat enemies. And he usually sends minions rather than fight himself.
Of course that's Mannimarco the mortal Necromancer. There's more than one being by the name Mannimarco after Daggerfall. Possibly as many as nine, but at least two and at least one of them "http://www.youtube.com/watch?v=4eisphnhri0&feature=related"
Of course as with all things Elder Scrolls lore there is another opinion. MK wrote http://www.imperial-library.info/content/nu-hatta-sphinxmoth-inquiry-tree and this was his opinion on King Of Worms:
King of Worms: "The Jills of Aka-tosh have mended this numidition. Mannimarco remains as he was: the high priest of maggots."The Jills of Akatosh are lady dragons who fix dragon breaks. What Michael is indicating here is that Mannimarco did not become a god at all at the end of Daggerfall and remained mortal.
However there is clearly a Wormgod in the sky in TES IV that creates Black Soul Gems. So perhaps a new god was created but he is not Mannimarco. Confused yet?