Question said:
DC 0? What kind of mitigating factors would that involve?
Extra Time and Additional Participants are the most commonly used, I think.
Time:
Takes an extra 10 minutes: -20.
Takes an extra 100 days: -200.
Participants:
Basically, it's -(2*spell level donated -1); so a Wizard-1 donating a 1st level spell slot gives you a -1 spellcraft DC. No RAQ limit on the number of additional participants. So if you have a Leadership score of 25, and half your followers are Adept-1's (NPC class - gets a 1st level spell at 1st level), and your Cohort can cast a 9th level spell, you get -17 for a 9th level spell from your Cohort, and -81 from a 1st level spells from each of your followers, for a total from that of -98.
Oh, and they stack; If the ritual takes 10 extra minutes, you get an extra -20 mitigating facot for the 11 minute ritual, for a running total on the above scenario of -118. If you can find a CR 6 or less outsider that can cast spells (Aasimar Wizard-5, perhaps), you can Permanently Summon multiples for more participants, to make later rituals even easier (base DC 110 for CR 6, DC 100 for CR 5, DC 90 for CR 4, DC 80 for CR 3, DC 70 for CR 2 or below), still without spending any XP or GP.
Getting dripping with cheese, starting up without Leadership at all, your 100 day, 11 minute spell gets a Spellcraft break of -220. Summoning a Couatl Permanently has a DC of 150 (base summon: DC 14, summons a CR 2 Outsider, +2 for each +1 CR over 2, Couatl is CR 10 Outsider, so DC 30; *5 for Permanent), making for a 65 day, 11 minute casting time (-130 from the days, -20 from the extra 10 minutes), and a Couatl has 4th level spells available. Each 4th level spell donated to your ritual is a -7 mitigating factor. Every two Permenently Summoned Couatl's is seven less days on Permenently Summoning the next (for your 0 day, - gp, 0 xp next researched Epic spell). After you've got enough that it's not meaningfully time consuming, you switch over to Planetars and Solars (both of which have 9th level spells available). This, of course, assumes that you're good-aligned. After a while with Planetars and Solars, not only do you have an army of high CR critters (A Solar is CR 22....) but they are utterly loyal (Summons have no choice), and have 9th level spells which can be drawn on to Mitigate whatever you want. A Permanent +100 Force Armor bonus to AC has a base spellcraft DC of 1030; with -17 for each Solar adding a 9th level spell slot, you need a mere 61 solars (which gives you -1037 in mitigating factors - you could drop one of them, and spend an extra 5 minutes casting - it's permanent, who cares). And then the next day, apply it to everyone in the party. Before Mitigating Factors, Vengeful Gaze of God has a spellcraft DC of only 619 (although with that many Solars helping you out, you'll want to tack on something so you can do this from home - Reveal and Transport, say, so you can zap someone from the comfort of your own demiplane - you've got room).
Mind you, if your DM house-rules that development costs are pre-mitigating factors, the cheese basically vanishes - the Permanent Couatl spell costs 1,350,000 gp, 54,000 xp, and 27 days to develop. You shouldn't be able to pull that off at all for a long time. And once you have, you won't be developing stepping-stone spells due to the expense involved; you'll be taking your 65 days for each Couatl you summon, until it's time for you to switch over to Solars. You'll miss out on adventures trying this. With that house rule. But it's a house-rule, and this is the rules forum, so......