Let's see... the as-written Epic Spellcasting rules don't include how to add actual spellcasting to a newly-created creature for ritualists. However, it's not necessary:
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Emphasis added, specific text from
Here), and the create critter option of the Conjour seed lets you add spell-like abilities.
So you use Conjor (where the rules for creating critters with spell-likes are found) along with Life (required), Fortify (required), and Compel (to make it loyal) to make a critter that has an Epic spell-like ability (what that ability is doesn't matter, as long as it can use it at least twice per day) that is utterly loyal to you. The creature only needs one hit dice.
21: Conjour (Base DC 21) is my base seed (no save, no SR); and I don't need to add anything to it not found in the base seed (besides the spell-like, which we'll get to)
25: Fortify (Base DC 17) is required by Conjour for giving the critter hit dice. I only need it to have one hit die (5 "temporary" HP), so I need 4 levels of temp HP augmentation for a DC of 25 from this aspect.
27: Life (base DC 27) gives the critter stats. I don't really need anything beyond the base, for this.
29: Compel (Base DC 19) needs to be in there. "Follow all the commands of your creator" is good, although I'll want the +10 for "unreasonable course of action" in there, for a 29.
60: I also need a spell-like ability, which can be anything. Maybe Life (DC 27) for what amounts to Ressurection, and Heal (Base DC 25, +6 for getting rid of ability drain, +2 for getting rid of negative levels, too).
So before adding Permanent, it's got a base DC of 162. After making it Permanent, it's 810 (which is a pity; Permanently summoning a Planetar would only be 210, but that's essentially disallowed here). Now we need to mitigate that to the point where it doesn't cost anything.
I use Lesser Planar Binding to get some allied ritualists.
Trumpet Archons are 12 HD Outsiders that cast as 12th level clerics. We can do better - Ghaeles are 10 HD outsiders that cast as 14th level clerics. That'll do. They've got a +10 Will save and a Charisma modifier of only +3, and SR 28. Which means if I've got a Will save DC on Conjouration spells of 30 or better, and can get a +22 or better modifier on my opposed Charisma check, I'm only looking at nat-20 and nat-1 clauses on getting these guys to do me a service. Moment of Prescience will give me a +caster level on an opposed ability check. If I've got an Orange Prism Ioun Stone, that's a a caster level of 22, for a +22 modifier on the bargaining (provided I've got a Charisma of 10). Sufficient. If he gets a 20, he's got a 23. If I get a 2, I've got a 24. I win. 90% success rate on the Calling. If I ignore any slots from a high ability modifier, a standard Wizard can do this four times per day with the full ritual. At a caster level of 22, I can keep them around for up to 22 days. The deal is that they help me as ritualists on my next 44 Epic spells. 88 attempts in that period, with a 90% success rate, averages 79.2 Called Ghaeles. Each casts as a Cleric-14, so that's 7th level spells, at 13 points of mitigation each. Oh, hey - that's 1027 points of mitigation right there from their 7th level spell slots. When it comes down to it, I really only need 62 of them (so a 70% success rate on the Planar Binding is enough for this; that'd be a save DC of 26 - as I can Heighten the Planar Binding spell to 9th, I only need a +7 Intelligence modifier to do that (+5, if I take Spell Focus and Greater Spell Focus (Conjouration)). With all those spell slots, it's a DC 0 Epic spell, for no research time, no XP cost, and no GP cost.
This is a 22 day warm-up time.
So now I'm mass-manufactoring healing critters, at a rate of 2 per day. If they're left running around after? Well, I don't really care, they won't hurt much, and they're easy to dispose of if they do turn out to cause problems. Plus they're fanatically loyal to me, forever. But each one I create has a 10th level spell-like abilities that I can use to fuel Epic casting, at 19 mitigation each, for 38 per day (initially). As creating one costs 810 base spellcraft, after I've got 43 of them (22 days after I'm done with warm-up, day 44), I can double up all my numbers and twin the Epic spell, still be at 0 DC. After another 43 of them (11 days after that, day 55), I can triple up my base numbers to make 3 of them with each casting. After I've got another 43 of them (8 more days, day 63), I can quadruple up my base numbers to make 4 of them with each casting. Oh yeah, and at this point, about two month's into my prep-work, I've got something like 3,200 points of mitigation readily available. And this is exponential growth - I double my available mitigation every 22 days this way, quite easily (it's actually slightly faster than that, but I don't feel like working up the spreadsheet at the moment to get precise figures).
How high of a DC is the "I Win" spell? You've granted me arbitrarily high prep-time. With a 22 day doubling period on the 22 day warm-up period for an 810 DC, in 220 days from "start" I get mitigation of roughly 14,580. In 352 days (22 day warm-up plus 15 doubling cycles), I get mitigation of roughly 26,542,080 points.