ur better off speed tuning when you have an actual team you know will work on all affinity, and when you do speed tune in the long run you will be dedicating some of ur champs purely to demon lord meaning they wont be much use else where ,mainly because when u speed tune some champs have to sit at the 180 ish speed mark to tune properly with the rest of ur team, and in later game u will struggle using a 18x speed champion in other high end areas of the game where that speed isnt high enough.

fahrakin, doom preist, toragi are all great champs you can use for it, not saying they are the ones u should use, but if ur going down the route of not wanting to speed tune for now then the best advice i can give you is you must have decrease attack active on the demon lord at all times as this is a must if ur not using an unkillable team, and add some survivablility such as increase def/ally protects/healing/shields etc burn/psn dmg is handy, fahrakin can play a pretty big role in ramping up ur dmg vs it.

and again if ur comp is not an unkillable one then you probably want be to aiming for around 50k + hp, 2700 (MINIMUM) def as standard on all ur champions that will be fighting it.

Others are going to be more helpful. You have some good survivability in your roster to build around. What team do you normally run and what damage do you normally put out on CB. Just out of curiosity?

Looks like you have more than what you're showing. Post the rest of your roster? There are several unkillable teams that you can build around Demytha and/or Seeker.