I'de go with a hellfire wyrm from the MM2.
It has a shapechange ability that allows it to "appear" like any devil threat you want it to.
High SR 37, but not impossible. It has sending and demand at will so it can be a well informed and well organised mastermind.
Hit them with demand every 10 minutes. Be creative with the suggestion, if the party doesn't act quickly they can be completely neutralised at a distance. The hellfire wyrm can do this from anywhere, another plane of existence, inside a volcano, every 10 minutes (per sending).
It will have the following things active: true seeing, improved invisibility, misdirection, unholy aura, sitting on unhallowed ground with a spell effect that pleases you tied to the entire area (I like dimensional anchor) and possibly active walls of fire. Don't forget the magic circle vs. good that gives it bonuses to saves in this area.
Remember to use the quicken spell-like ability feat to it's utmost. In the MM2 that feat allows the monster to quicken EACH of its spell like abilities 1/day.
So as soon as your PCs pop up:
Quicken blasphemy to stun lock them + flyby attack breath weapon + infernal aura.
Then blasphemy to stun lock them + quicken summon a gelugon + infernal aura damage.
If the blasphemy/fear is working well then it should just use its action to keep them stun locked and have the gelugon beat the stuffing out of them.
If it isn't working then switch in a quickened greater dispel.
Otherwise moderate success would have the gelugon cone of cold + fear aura, the hellfire wyrm breath + infernal aura + quickened fire-storm.
Greater dispel any items or characters that seem to be working too well and quicken a lesser ability like hold person, wall of fire or pyrotechnics, have the gelugon isolate PCs with walls of ice or keep hammering away the cones of cold. Breath weapon whenever possible as 7d10 of the fire damage cuts through all resistances effectively.
If they start to win, teleport/fly away and shapechange into a fiend that regenerates (pit fiend for instance), heal up to full and return to the fight.
Quickened greater dispel on a wizard then a full attack with some decent power-attack works well too. Dispel magic weapons, nice chuncky legacy weapons and staffs of power.
You could even start with dispelling rather than damaging spells.
Should make for a hell of fight.