There's some ways to make half the damage divine and irresistable, or to change the damage type to a different element entirely. Do those count for you?
I had been looking for similar abilities like the irrisen Ice and Snow witch... dass to houserule for other Elements, Guess thats why paizo didnt publish some.
Benthic Spell (Metamagic). Turn the spell into bludgeoning damage.
That doesn't really help me with that fire or acid immunity, though. The point is to deal fire or acid damage to a fire and acid immune character, not to deal a different kind of damage.
True. But immunity is immunity. Sometimes you have to work around a problem, rather than expend so much resources in defeating it.
The ability for winter witches to overcome elemental immunities is unique, as far as I know.
An exercise in futility, I’m afraid.
There are ways to curse someone with vulnerability to fire, though it doesn’t make them lose their fire immunity, so again an exercise in futility. Its like trying to dig through an adamantine wall with a spoon. Sure, you can do it with enough (godly) strength and time, but why just not use a better tool?
Are you the GM or a player? If the latter, how long do you have ingame to work on this? You could always research a spell that specifically does only 1 hp fire damage but bypasses fire immunity---or if you're just trying to shut down someone's regeneration, a spell that does 1 hp untyped damage that nonetheless shuts down any regeneration for a round. It'd be pretty high-level, of course.
The half divine effects make the divine damage not fire or acid, so that's out by your criteria. D&D 3.5 had a feat which would suit you but it was intentionally not repeated in PF. The name was searing spell & it made half the fire damage beat fire immunity (but still be fire damage), & was +1 spell level. In Sandstorm which wasn't on the SRD.
I can't help w/ overcoming fire immunity, but from the Advanced Race Guide (p. 128), an Ifrit Immolator (Inquisitor) has a particular Judgment (Immolation) that gets increasingly better at lowering fire resistance.
Plus on the following page (129), you get an awesome picture of an Ifrit Immolator. So awesome I'm planning a future PC around it...
hellfire?
i am assuming you need to kill something with regeneration/acid or fire that somehow also got fire and acid immunity.
the good thing about hellfire is that it is one thing. even the evil part is also considered fire for all aspects "Hellfire is treated as normal fire". so creatures who are immune to fire damage but not the evil damage still are considered to take fire damage for stopping regeneration and such.
there are many ways to be able deal hellfire. the spell hellfire ray for one. the diabolist prestige class specialize in the ray and other forms of hellfire.
also if the problem is a regenerating fire immune creature, as i assumed, then things that stop regeneration or nullify it for a time should also work. (there are class abilities and spells that do just that -stop the regeneration of a creature to allow killing it with normal means).
Kineticist's Kineticist's Draining Infusion lets you damage creatures whose elemental subtype matches the element you are blasting with. It bypasses damage reduction, resistances, and immunities, provided that the target fails its Fortitude Save. It won't work on Devils (immune to fire, but not having the Fire subtype) or Demons (immune to Electricity, but not having the Electricity/Air subtype), but it will work on some other things. Although something that is weird enough to have Regeneration/Acid or Fire AND have Acid and Fire Immunity might well be equally weird . . . .
(Also, not exactly what you were looking for but related, Searing Flame Utility Wild Talent eats away fire resistance.)
A lot of Augmented Mythic spells have the ability to ignore resistances and immunities, but that's probably out of reach unless your GM's running that kind of campaign.
Can you please clarify, is the point of trying to deal fire/acid damage to the immune creature that they also have regeneration?
If that's the case, you should look at alternatives to suppress regeneration, of which there are a few as opposed to trying to defeat energy immunity.