DC 45 with Normal Stuff
(credit to Ben S. for formatting some of this content)
Circle of Death is a 6th level wizard spell, so its base DC is 16 (10 + spell level).
Since you can obtain a 20 Int at character creation (18 base + 2 racial bonus), you can have 23 base Int base by 12th level. That adds +6, for a DC of 22.
A headband of vast intelligence +4 costs 16,000 gp, and provides another +2, for DC 24. If you can get better than +1 DC per 20k gp for the rest of your gold, go with this. A headband of vast intelligence +6 costs 36,000 gp (over 1/3 of our total wealth before multiplication!), and gives +3 to DC 25. Since I couldn't spend all our gold on DC boosts, I have us use this one instead.
Spell Focus (necromancy) and Greater Spell Focus (necromancy) add another +2, for DC 27.
A Void Shard costs 250gp. Using one as a power component to enhance a necromancy spell like Circle of Death imposes a -2 penalty on saves, for an effective DC of 29.
A Whimsy Star costs 200 gp. Using one to enhance any spell has a one in six chance of raising the spell's level (and thus its save) by one, to 30. Alchemical power components are not limited to one per spell, but their effects don't stack. Fortunately, since neither of these components directly affect the same thing, they essentially do stack, at least for our purposes.
Djezet can be used as a yet still additional material component for an effective +1 save DC (via +1 actual spell level). This costs 1,200 gp per casting, and gets us to 31.
Yliaster is yet another additional component we can add, this time it's called a 'rare esoteric component', which again adds 1 more to our DC. It costs 1,200 gp per cast. It also adds 2 to our CL, but that isn't particularly relevant. This puts our DC at 32.
A Focused Metamagic Rod costs 11,000 gp and adds +2 to the save DC for one target of a multi-target spell (like Circle of Death). This bumps us to 34. Rather than using the rod, though, we are going to actually want to take access to the feat as one of our two free metamagic feats via Sacred Geometry. We can't boost a spell higher than the highest spell level we can cast, but thanks to Djezet that's 7 not 6 so we can totally apply Focused Spell via Sacred Geometry, since Sacred Geometry's limitation only cares about the effective spell level of a particular pile of combinations (that is, the 'base' level of a spell) not the actual level we are casting it at (otherwise, since it changes all metamagic to +0 adjustment, there would be no limit at all).
A Persistent Metamagic Rod costs 32,500 gp and forces opponents to save twice against the spell. That's different from a straight bonus, but mathematically it works out to an average roll of ~7 on a d20 instead of 10.5, so you can think of it as increasing the save DC by 3, to 37.
Summon Evil Monster lets us summon a pugwampi as a standard action via Summon Monster II or similar. Pugwampi are amazing and force everyone who doesn't have a luck bonus (and isn't a gnoll, gremlin, or animal) to roll twice and take the worse result on every roll. In conjunction with Persist Spell, this means our opponent is rolling on average a bit worse than a 5, and we can treat our DC as effectively 39.
If we are an Exploiter we get access to arcanist exploits, which increase our save DC by 2. Losing our school can be fixed by taking the school understanding exploit. Losing our familiar, however, requires us to take the Familiar Bond feat which makes us need the Iron Will feat. We take House of Green Mothers Pupil instead (traits are cheaper), but that would conflict with Insistent Benefactor if we were able to go the Toxic Spell route later. The exploit gets us to 41.
Tenebrous Spell can get us +1 save DC but it's another +1 spell level adjustment unless you combine it with Umbral Spell which is a +2 level adjustment. Instead, we will risk a harmful Wild Magic event to apply the metamagic feat without costs by using Wild Magic to boost casting by making a DC 21 caster level check. We are now at 42.
The demonic corruption corruption's demonic enhancement manifestation grants a +2 profane bonus to an ability score of your choice. That raises the DC to 43
Nocticula can give us another +6 Profane bonus we could apply to Int. That would be better except 1) we have no way of making her do that and 2) she can kill us at any time afterwards.
A regular succubus could give us a +2 and we can summon them, but that's not better than the corruption and it doesn't stack.
If your character is of venerable age, they get a +3 bonus to Int. This would improve your save DCs by another 2, because we have an odd base Int, getting us to DC 45.
We can get an extra feat from being an antihero, since we aren't really gonna benefit from extra actions or other hero point things on our save DC.
We can also get an extra feat from being human.
We can also get an extra feat from paragon surge via either just casting it normally or a scroll of it that's the race-agnostic self-realization domain version. This gets us exactly back up to the required number of free feats.
Since we have to be a human, we can't be a gnoll, which sucks, so we have to take a luck bonus. A luckstone costs 20,000 gp. Why couldn't it be two free feats? Gnolls never get to be the answer :(
Summon Evil Monster
Greater Spell Focus(necromancy)
Doesn't use any bonus feats, but if you don't take Time Stutter for a PVP fight idk what you are doing.
Uses 88,500 of our 162,000 gp from WBL+Crafting bonus (we get scribe scroll at level 1 for free). Costs 3,450 gp per casting in a wide variety of rare components. At least some of the remaining wealth is spent on a rod of quicken spell I assume, and probably a pearl of power or two.
Our 1 round prep involves casting psychic asylum so we get 15 minutes prep instead for, like, personal buff stuff and whatnot. We also cast a buffed-up Summon Monster IV of some sort to get 1d4+1 pugwampis.
DC A Lot with Less Likely to Work Stuff
Toxic Spell is also a +1 level adjustment metamagic we don't have room for. If we did, we would ordinarily use it with the poison of the Mantis God to give it a DC 45 (35 for creatures immune to poison) poison effect that bypasses poison immunity and inflicts 1d2 negative levels. We can't do that either, both because debuffs are forbidden and because Achaekek is part of Paizo's 3.5 material. We can use the feat on a different fortitude save based spell-- increasing the DC with Insistent Benefactor-- and with different poisons, but that's still debuffing the target so the -(2d6+3d4+2d3+1d2) Con and instant day-long not-mind-affecting domination and hour-long paralysis and -(1d3+1d2) Dex from a Toxic to the Eighth Blend With Surroundings cast via Sacred Geometry with Djezet for 6 toxic plus a metamagic gem for 1 toxic plus 1 toxic base from taking the feat the normal way and applying Wayang Spellhunter to it is a no go.
The skinsend spell lets you become a construct, something your GM seems to be specifically trying to avoid given your list of allowed races. Constructs can get their ability scores increased by as much as you want via mods. This costs 5,000 gp per +2. Unfortunately, it also takes 5 days per upgrade, so you will need to be casting skinsend a lot. Fortunately, crafting is 8 hours of work per day, so the duration of the spell is sufficient for work to be performed, and the text of the spell is very clear that it is the same skin that is put forth each time you cast the spell. If this is permitted, we can spend almost all of our gold on exotic skincare products-- only the consumable optional additional components do better per gp for DC-- and get +62 Int, for a total spell save DC of 70 accounting for all the other stuff we then can't buy and all the other stuff we can still have from class features and feats etc. I assume, however, that we can't do this because it's pretty clear your GM wants no constructs and this is probably why.
If we take the Void school (or school understanding), we can debuff a target's saving throws by half our caster level for a round as a standard action. Using a hero point (gained for example from a scroll of heroic fortune), we can take this standard action and still have time to Circle of Death. Buffing caster level is really not very hard-- there are lots and lots of compatible ways that just cost gold. However, you specified no debuffing the target, so we will ignore this.
If you could somehow surprise your opponents, the Ambuscading Spell feat would give them a penalty on saving throws in the surprise round (-1 or -2, depending on whether they've acted in the surprise round).
If you cast the spell into a greater glyph of warding (you can get this spell on your spell list if desired in a variety of ways, most probably by being a Samsaran with Mystic Past Life) you can cast improve trap on it. That increases the save DC by +2, but it means you gotta put it in a trap first.
The Trap-Stealer's Rod allows a character to deploy a stored trap once per day at any time. An automatic reset proximity activated alchemical trap mimicking the effects of Circle of Death but with infinity DC and DC 0 to spot it and DC 0 Disable Device to disarm it costs ~101,000 gp. The rod itself costs 13,500 gp. This means you have 47,500 gp left over to spend on whatever you want. Infinity is the biggest DC, particularly since it can be whatever infinity you want, as long as the value is bigger than or equal to '30'. This is very close to making custom items, though, so it is probably not allowed.
Primal magic can force creatures to roll twice and take the lower result.
The Eldritch Researcher story feat would work great, but that requires being the person who invented circle of death, and that seems unlikely to fly unless you have a Scepter of Ages or something. If your GM does allow it it boosts DC by 1 and lets us apply an extra +1 metamagic for free.