This build is adapted to all difficulty settings, including unfair. The goal is to be able to crown control up to 4 demons at the same time, even the most powerful ones, reliably, despite their very high Spell resistance and saves on hardest difficulties, while still full attacking each turn (if you control 2 demons max.)
Level 1 : Sanctified slayer (inquisitor subclass), Take divine favor.
Elf (the +2 bonus to overcome SR is great)
i took nobility domain because of the level 8 power who stacks with most of the others buffs, as an insight bonus, but it will come late, feel free to take whatever you want.
Acolyte give WIS to persuasion instead of charisma. you’ll want to take persuasion, perception, trickery (there is some very good items you can get with a good trickery while you’re alone), knowledge arcana because you’re very good at it and you’ll get some extra-points with high INT, you’ll need lore : religion 3 for mystic theurge.
Level 2 : Sanctified slayer
Level 3 : Sanctified slayer, precise shot, precise strike (who gives you a sneak attack dice).
Level 4 : Sanctified slayer, +1 DEX (you’ll get level 2 spells), you gain another sneak attack dice.
Level 5 : Wizard (i took a basic one with hare for +4 initiative +2 perception and divination school for another initiative bonus but do as you wish). Take weapon focus (short or long bow) for 1 extra hit and Spell penetration as the mage feat. huricane bow and reduce person are nice lvl 1 spells.
Level 6 : Wizard.
Mythic level 1 : Force reality, abundant casting.
Level 7 : Wizard. Rapid fire, Sense vital for more sneak attack dices and scorching ray are nice.
Mythic level 2 : Rapid Shot, mythic (to cancel the -2 penalty)
Level 8 : Mystic theurge +1 INT
Level 9 : Mystic theurge, Greater Spell Penetration, (haste, and for later battering blast)
Mythic level 3 : Aeon, improved abondant casting, enforcing gaze - caster level
Level 10 : Mystic theurge : you’ll gain your level 3 inquisitors spells : litany of eloquence is the thing.
this spell have an obvious flaw, a short duration, but 2 big advantages, the first one, there is no saves to prevent the effect, only spell resistance work, and even better, it can be used as a swift action, allowing you to do a move action + a standard action (like casting a spell) + a swift action (litany of eloquence). or a full attack + a swift action. you can also use litany 2 times in one round (one as a swift action, the second time as a standard action).
This spell works on most of the enemies like slumber from a witch (not only demons), only very few are immune to it (swarms, golems, few bosses but you can control adds and focus boss)
At this time you should have found 2 Lesser extend metamagic rod, allowing you to use litany 6 times / day as a 2 round duration spell.
Level 11 : mystic theurge, Extend Spell (Metamagic), you’ll get level 4 mage spells
Mythic level 4 : Aeon, mythic spell penetration.
Level 12 : +1 DEX mystic theurge, you’ve got “combined level 3” to allow 7 extra cast of litany in the level 4 mage slots.
Level 13 : mystic theurge, improved initiative to make sure you’ll play first nearly all the time.
you’ll get level 4 inquisitor spell allowing you to use 6 extra cast of extended litany (with the metamagic feat)
i will not explain up to level 20, you’ll be able to cast 7 more extended litany witth your level 5 mage slot for nearly 30 cast per day in total, half of them with extented metamagic feat and more than half of the rest with also 2 round duration, thanks to extended metamagic rods. “abondant casting mythic abilities” are so good for mythic theurge.
Don’t forget you can use all rays spells reliably because you’ll be good at hitting with ranged touch attack and spells with attacks rolls don’t have saves, and SR is not a problem for you.
you can take whatever you like as a feats from now.
Don’t be afraid with the 2 round duration of litany, even big boss fights tend to last only few rounds, the enemies can wipe your party in 2 or 3 round on hardest difficulties, but the opposite is also true with an optimized party for higher difficulty.
you’ll be able to overcome SR nearly all the time even on unfair.
your base caster level will be (mage level + mystic theurge level for mage side and inquisitor level + mystic theurge level for inquisitor side).
and your roll to overcome SR :
1D20+ base caster level + 2 (racial) + 4 (normal and greater spell penetration) + up to 10 (spell penetration mythic) + up to 10 (aeon bane) + up to 4 (enforced gaze - caster level) + INT or SAG modifier + few magic item who can boost that.
Even enemies with 50 SR can be overcome without too much trouble
On top of that, Aeon have some very interesting spells. i only mentionned some key spells, but you’ll get very good flexibility, lot’s of good buffs spells like transformation to get full BAB, etc