Slaying an Epic, possibly Immortal, Dragon

[Another_Lurker]

I'm a player in an Epic campaign building to full godhood, and I've set my sights on removing one of the party's competition.

Said competition is the eldest chromatic dragon (a Red dragon, specifically) in the setting. Some of the things my GM has said hint that he and is Metallic counterpart are immortal beyond a dragon's lack of maximum age.

I don't forsee us having terribly much trouble with the fight itself; the party consists of a Psychic, a Witch, an Arcanist with some Words of Power via Feats (Witch's Cohort), a Nightmare Dread, an Herbalist Alchemist (Dread's Cohort), a Vital-Striking Titan Fighter, a Bushi Warder (myself), and a Stalker (my cohort). My bigger concern is making the kill stick.

I have a rutual that when applied will remove the target's soul and siphon it into a prepared vessel, birthing an Artifact. The only problem is that the target gets a Will save to resist, and the base DC on it is utterly pitiful for this task- 15 plus the caster's int mod. Right now that's total DC of 25, which I'm absolutely certain he can't fail except on a 1.

What I'm looking for are as many effects (preferably stacking) as possible that can either:
A) Increase the flat DC of an ability
or
B) Reduce a target's Will save for at least the 1-minute cast time on the ritual.

I've already got a couple things in the works that will help out with my int score.

The base game is Pathfinder 1e, Path of War is in use, DSP Psionics are in use, Occult Adventures Psychic Magic is in use. The Jesse's D&D Epic Pathfinder Handbook is in use. Other 3rd party content is available with GM approval, 3.5 content is available with GM approval.

We're still a decent ways off from enacting this plan, but the better we can plan the more likely this actually works.

[Magelyte]

For reducing a creatures will save, there’s mind fog for a -10 to will saves and prediction of failure for a no-save shaken and sickened. You could also try reducing the dragon’s wisdom score, either by ability damage and drain or by using baleful shadow transmutation to turn it into something with a low wisdom. Negative levels are also a good way of reducing saving throws, if you can incapacitate the dragon you could have a wight or something similar energy drain it until it has almost no chance of passing the saving throw.

[upho]

It's probably worth noting that that a greatly improved DC likely won't be enough if the dragon has mythic ranks/tiers, since that may very well grant it mythic abilities allowing for save rerolls and/or augmented spells with similar/greater effects, such as mythic wish. So your party will probably need to complement an improved DC with stuff able to change the dragon's altered and/or rerolled save more times than the dragon is able to, such as by having two or more party members being able to cast mythic wish (allowing for dictating the die roll for the dragon's save as an immediate action).

[noob]

You can stack mind fog and lesser wish for a total of -17.
add up like 20 negative levels and you did lower the save of -37.
Then you top that with wisdom drain to 0 wisdom.

By the way Upho the starting point is that they made the target be unconscious so the target is not using wishes at all.

[Nizaris]

Ill-Omen is a good no-save option to force disadvantage on their next d20 roll, just need to time its use for improving the odds of a low roll or Nat 1.