Banishment
For Cleric, minimum Will Save of 19 (10+3+6), not too difficult for the Pit Fiend to break (in fact impossible)
For Wiz/Sorc, min Will Save of 20 (10+3+7), again not too difficult for the pit fiend to break.
Not too far fetched to assume that the spellcaster by 16 level has somewhere around 18 in their prime spellcasting ability, no? That just bumps each of them up by one however, not much. But again, it's just assuming an 18 in their stat.
Also, with banishment, you actually have an interesting Focus component, one that also results in the DC being 2 higher (and provides a +1 to breaking SR). So we have a 22 Will DC (and 17+d20 to break SR) for the cleric, and a 23 Will DC (and 17+d20 to break SR).
Still pretty much impossible.
Now might possibly be easier for me to make the example with the wizard (Because they are more likly to be stat focused on simply int). Wouldn't be too unreasonable to assume a +4 Headband of Intellect (shuffles the DC up to 25). Typically maybe a +1 inherent bonus to a stat, which is ineffectual mostly.
Feat wise, it's silly to assume a spellcaster would take Spell Focus : Abjuration (a cleric/exorcist might), and Spell Pentration in general isn't seen as being as useful. A heighten feat might be had, but it isn't too likly to be prepared with Banishment.
Now taking into account the Dismissal adjustments (which I'm not sure actually apply but it's fairly minor), -MonsterHD + CasterLevel... result -1 to the save, so save DC of 24.
Mediocre spellcaster has a 30% of breaking SR, and a 10% chance of getting the Pit Fiend to fail their save. About a 3% chance for putzy wizard to 'defeat' the Pit Fiend in first round, with no special prep (beyond preparing Banishment and having something Pit Fiend fears). Not much of anything, eh?

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Assume Putzoman is slightly prepared and has 2 or 3 items that said Pit Fiend doesn't like (like oh say, the fighter's holysilver sword, the cleric's holy symbol, and let's say a some holy water. -sidenote, I'm intentionally being putz about the items, although they are valid choices given the current Banishment description). SR check changes to 19+d20 (+2), the save dc jumps up by 4 (to 28).
Now MPutzoman has a 40% chance of breaking SR, and a 30% chance to get past the save. So about a 12% chance of defeat. Not too bad for one spell

. Of course, this assumes a wizard who had an 12 (4 =13, 8=14,12=15, 16=16). int. Using standard matrix, shuffle the starting int up to 15 (pretty much just a +1 to save DC, maybe a +2 given the assumed inherent bonus above-saveDC 30).
Putzo has the same 40% chance of beating SR and 40% chance to get past save, a simply 16% chance.
Not a real 'excess' of preparedness, and a 'reasonable' chance to deal with the Pit Fiend in one shot. The added benefit is Banishment would work doubleduty to clear out the summoned fodder mister Pit Fiend leads with.
Sidenote, unless Summon Baatezu gets majorly changed, if said Pit Fiend uses a non-targeted Dispel, some chance of him sending his buddy back to the Nine Hells.
Only real level of 'preparedness'/specialization is from the Focus components of the spell, and actual continued preparedness in that respect continues to raise the chances of poofing (5% on SR, 10% on Save DC, possibly double if your DM lets ya find a rare focus). Although, one thing I see happening, is that the monsters SR is slightly more significant than the normal, and may make the Penetration feats more desirable.