[3.5] Make this encounter fun

[Bluto]

So, the party I'm DMing for is about to do battle with a pit fiend and a blackguard.

They are a lvl 15 party, with six members, and not what you would consider optimized, but still geared up pretty well, and have had no problem taking on encounters above normal CRs for their level.

Looking over the unmodified stats, their melee fighters ACs are high enough that they will only get hit 10-15% of the time, and the fiends AC is similar enough compared to their attack bonuses.

Now, the fiend could just spam blasphemy, and let the blackguard slowly whittle the party down, but where's the fun in that? I'm thinking I will make the modification to Blasphemy from at will to 1d4 rounds or something, and let them have a go at it.

Tactics I 'm expecting to see are Lower Spell Resistance, Scale Weakening, and plenty of smite attempts, with 2 paladins in the party. I know they have plenty of fire resistance among them, or outright immunity, so the quickened fireballs and meteor swarm will be busts, though the fiend will throw them out, and he has no knowledge of their resistances, at least to begin with. I plan on summoning in a horned devil early, to give one more target, and a small threat in it's own right.

The blackguard will probably end up using his Improved Sunder on some of the melee fighters shields, which will scare the heck out of them - because a) loss of loot! and b) massive AC bonuses due to shield spec feats and abilities.

So, 2 questions - Tactics to make this encounter fun but challenging (A death or two is not a big deal at this level, but I don't really want a TPK.) If they get the better of the fiend, no big deal, it has plenty of escape routes.

and secondly, is the nerfing of the Blasphemy enough to lower the CR of the fiend?

[Flickerdart]

A Pit Fiend paired with a Blackguard would likely change his tactics from the default - it will be much more effective to send in his minion to debuff enemies with his aura, and then use targeted save effects, that it would be to keep the minion out of the way and use AoEs. It will probably use the same opening move (Fireball some stragglers, noting that it did no damage - at Int 26 with Spellcraft trained, it's easily smart enough to put 2 and 2 together - and then Mass Hold Monster everyone close to the Blackguard). The Blackguard's primary job will be staying near the PCs and keeping them from advancing on the Pit Fiend, leaving it free to spam its powerful SLAs, so give him a spiked chain, active Bull's Strength, Improved Trip, the works. If they don't know the game well, it'll make them think that the Blackguard is the melee muscle of the group, and they should try to coordinate some tactic to send a monk or something to try and take the Fiend out. At which point full attack, constrict, poison, the poor guy is bleeding on the ground.