He doesn't have to come anywhere, he just needs to strengthen the main DPS of your group. Most often it will be Arushae/Lann or Ember (not counting the main character).
Spells like Hearth and Inspiring Command are mainly cast before a fight (you can do this easily in most fights). In the end he will have Domain Zealot anyway.
Mythic evocation is pointless, he is not designed to cast offensive spells and archon aura is not enough of a gain to make this a worthwhile mythic feat (especially since the +2 to spell dc doesn't do anything in the late game anyway).
It is better to help the rest of the group to make sure they will be able to hit the enemy. As long as you are able to hit the enemy regularly, two or three characters will be able to easily kill anything important.
The interesting thing about some of the hardest fights in the game is that you do have other main characters in the story fighting with you, so you could set up a defensive perimeter and let the fight come to you while they bear the brunt. That may be how they're designed to play out. I've just found it more effective to take the fight to the enemy and eliminate them before they can do bad things, as with the Regill shot I originally poasted of him taking Darrazand down in one round with Touch of Law guaranteeing all hits.
Mythic Evo is not pointless. I've poasted tons of pics with Archon's malus. Key thing about Archon's is that it debuffs Saves, Attack, and AC. There are a few bosses that have decent odds to make the save but it's golden to soften up adds in fights like Abode. Shaken and Sicken also debuff saves which increases chances of Archon's getting there and even Seelah's Archon's forces a save that can be crit missed or affect adds if Sosiel's misses. There are two different items that buff evo dcs available before Fane and they do work on Archon's. If you want you can swap them out after casting it since Archon's is long-lasting. There are several other Evo spells that are very good in this setting (Sound Burst, Arrow of Law, Holy Smite, Arbitrament) that work well against non-bosses. Nenio's Illusion control spells can gum up the battlefield while Sosiel focuses on particular threats. Putrefaction even debuffs Will saves via WIS drain.
On Unfair that takes high level play but any other difficulty it's gold with either Sosiel or Daeran.
I want Domain Zealot immediately to turn on Touch of Good/Luck in combat doubling his action econ. Mythic Evo turns on Archon's, Sound Burst, and Arrow of Law, Abundant Casting lets him buff and cast both, then Mythic Spell Pen is such a big convenience. Improved Abundant, Improved Initiative Mythic, then Impossible for Hearth works well.
You can get Inspiring Command from Regill if you want it, although other Insight bonuses start showing up eventually. I used to multi Lann into a class with a Domain to get it there.
If you don't like Luck and Good Domain I'd just respec him into that new Cleric Class to get the ones you want, and make him more resilent in close combat if you want to add Madness. What are you typically using Madness for?
