How to maximize success rate of Phantasmal Killer/Weird in realistic scenario?

Question

Expanding upon a title on question ramifications:

• Game is RAW, the only no-go department are Wish shenanigans.
• Arcanist, currently level 8.
• Save-or-suck/Save-or-die focused build, mostly control mage.
• Downsides of PK: two saves, Will and then Fort, both of which must be failed for spell to kill the target.
• A maximum of 3 turn sequence, preferably 2.

What partial solutions to the problem I have so far:

• Maximizing Int to increase DC (+9 total, assuming +6 Int Headband),
• Witch Cohort using Evil Eye > Misfortune sequence > Bestow Curse if 3 turn sequence is to be used (that's -2 to saves, disadvantage and another -4 to saves, but assuming all of it will go through is extremely optimistic).
• Bloody Tears and Jagged Smile buff on Arcanist before any encounter starts (+4 to DC).
• Potent Magic Exploit (+2 to DC).
• Vision of Hell (for a total of -4 to saves).
• Persistent Spell Metamagic Rod (disadvantage on saves, stacks with misfortune).

For now, extremely optimistic scenario, it's a:

• Sequence of Evil Eye + Vision of Hell > Bestow Curse + "something" > Misfortune + Phantasmal Killer. Middle turn of the sequence is one I'd love to skip in favor of more pre-combat buffs and constant buffs.
• DC of 10 + 4 + 9 + 4 +2 = 29 DC total.
• Saves with - 2 - 4 = -6 penalty (-2 without Bestow Curse) and double disadvantage.

...but I don't want to count on extremely optimistic scenarios, which is why I'm looking for more options to boost Phantasmal Killer/Weird chance of success.

I'm primarily interested in answers that revolve around levels 8-12 but options for later will be a nice addition too. Disregard the SR issues and counterspelling issues. I have solutions to those roughly planned out already.

Answer 1

Sacred Geometry

Sacred Geometry is a little silly, but it works. If you've ever played the game of 24, you'll have fun. In short, take Sacred Geometry (Familiar Spell, Cherry Blossom Spell) and a bunch of ranks in Knowledge (Engineering), and infuse your familiar with Familiar Cherry Blossom Rays of Frost, Magic Missiles, or Acid Arrows, depending on your maximum castable spell level, for the cost of only 0th/1st/2nd level spell slots and spells prepared. Your familiar will then cast them at your victim, forcing fortitude saves (fairly easy, but you've stacked on a bunch of penalties by that point) to not lose 2 points to all mental or physical stats. Your witch can presumably take the feat instead of you. It's not ideal for several purposes, but it does stack on a little extra penalty - especially with acid arrow's damage every round. If you're taking Sacred Geometry, please, please read it carefully.

Potent Magic

This might be obvious, but Arcanists have a very useful feature that I didn't see in your question: spending an arcane reservoir point to increase DC by 1. If you care a lot, you might want to pick up the Potent Magic exploit, which increases this by a further 1.

Shadow Subdomain

Oddly, this subdomain for the Darkness and Death domains doesn't seem to be on d20pfsrd, though it is on Aonprd. Regardless, it is fantastic: 3+ times a day the user can reroll any nearby saving throw against an illusion or necromancy spell - fail or success. This could be the will or fortitude saves, I believe. Domains are a little tricky to get, so you'll need to either spend one of your cohort's levels on Cleric (not as bad as it sounds, domains are great and variant channeling is interesting, not to mention Idealist for some enhanced spells due to planar traits), or take Believer's Boon on the witch for 1/day use. Since your witch presumably has a familiar, an Emissary familiar might be more to your taste, also giving a 1/day use.

Arcane Concordance

It may be a bit pricey, but a wand of Arcane Concordance (or just a friendly bard) will give a final +1 and a few other goodies. Not a bad thing to have in your back pocket.

Fool's Gold

This one's a bit of a stretch, but the Fool's Gold spell (no, the other one) will give the foe another -2 penalty if it fails a save (again, an easy save, but with all the penalties you've already stacked on top...). I'm not sure how you want to plant it on your foe's person, but with enough ingenuity I'm sure it's possible. Familiars as always? Apport Object? Quickened Apport Object? Sacred Geometry (Familiar Spell) Apport Object?

Answer 2

In addition to those listed by Phoenices:

Weapon Trick: Ranged Weapon

This is admittedly yet another thing for your allies/cohort to do, but -4 is -4. Requires another feat as prereq.

Riving Strike

Again something for your ally, and again needs another feat. At least it stacks with the Weapon Trick if you can eventually get both.

Alchemical Power Component: Silver

For 10gp of silver, boost the DC of the disbelieve half of the spell by 1.