Oh, this one's a classic. You have a character with an ego so large, it's on the verge of collapsing into an intellectual black hole. You want to make it into a comic relief but instead, end up with an irritating pain in the a*** that ruins the mood.
Such character would be Mephistopheles, a shady creature, serving as the quest-giver and a complete rip-off of Christian Weston Chandler. So far, I avoided the main irritating elements of your average narcissist, by replacing them with the weird goals and delusions, like believing that God spoke to him in his dreams and ordered Mephisto to abolish all religions and replace them with atheism in God's name.
Of course, I can't keep that up forever, that would be just too unrealistic, less hilarious traits will crop up and I have zero ideas on how should I handle them.