See now if you has read what I was saying regarding the Pit Fiend, then your entire question here would be answered. But more to the point, why are you assuming that the Dread Necro has a bunch of no save, no SR, crippling spells at their disposal? Because undead creation aside, the Dread Necro doesn't seem to have much in the way of anything in terms of no SR offensive spells. Hell, there aren't a lot of no-save spells available either, so that Dread Necro had better have some specialization in overcoming SR and saves if they want to be useful beyond the latest undead pet their can boast. Seriously, if you are a Dread Necro facing down a Pit Fiend, how are you taking it out in a round or two, in this supposed no save, no SR method you speak of? Further, while you might be somewhat protected from fire (unless you have immunity (presumably from some source other than the abilities available to your class), 10d6 fireballs can do up to 60 damage, and Energy Resistance typically doesn't go past 30. I would say doing an extra 0 to 30 damage every round with an action you aren't using anyways, to the jackass that just paralyzed you with Dexterity damage its a terrible plan, especially if this is a person who is likely now also paralyzed (as per Hold Person or Power Word Stun), and one of the more squishy targets of the group. And even if you assume you are protected from fire damage, are you going to provide the additional cost of giving fire immunity to every undead minion you are taking with you? You don't have access to the spell itself, so are you going to pay for an (not inexpensive) item for each of your minions to get this? And this is all assuming the Pit Fiend actually feels like saying around as opposed to, oh I don't know, teleporting away to have those undead minions you mentioned get to healing his ability damage, only to come back rather pissed off, and probabl with you still paralyzed.
As for what this has to do with adept vs swordsage, it is tangentially related. I had, a while back, asked why exactly the swordsage was supposedly outclassed by pretty much all of tier 3. This lead to my questioning how exactly the various spells of the tier 3 spellcasters makes them that much better in a fight than the Swordsage, the "HP damage sucks" answer was given, and a long discussion resulted (involving an illusion based pit trap beyond the means of most any tier 3 caster). This lead to a discussion of save or die/suck spells vs. direct damage, and I was using the Pit Fiend as an example of a high level monster with saving throw bonuses beyond the DC of most save or die/suck spells. (And the Pit Fiend isn't even the best of those in its category at resisting such spells). then shivering touch was mentioned, hence why a paralyzed pit fiend came into play. Understand?
Alright, I shall address a few problems I see with the spells you mention, in order of the spells you mentioned. Don't get me wrong, these are all good spells, but are potentially hindered in this case:
1. Ray of Exhaustion- Save makes this fatigued, failing to credibly take out the target before it has the ability to at least paralyze you for trying. Allows SR.
2. Vampire Touch- Direct damage. Kind of missing the point of the discussion. Assuming you get reach extension this is, at max, 90 hp worth of damage at level 20, within 30ft. May put you out of reach of Power Word Stun. Won't help when you have hold person cast on you in the next round. Will help soften it up a bit though. Allows SR.
3. Enervation- Max 6 negative levels, weakening it maybe, but not taking it out of commission. Allows SR. Also, puts you within 35 feet of the Pit Fiend. (Notably, only reduced HP by 30, not putting the Pit Fiend within range for Symbol of Death)
4. Black Tentacles- Max grapple for this level is +28. Pit Fiend has +35 to grapple, and can either fly or simply move out of the area on the next round (assuming teleportation has been neutralized, requiring the aid of other party members)
5. Wave of Fatigue- Might be helpful if quickened and coupled with Ray of Exhaustion. Strong debuff, bot not enough to neutralize the threat, if both pass SR.
6. Cloudkill- Creatures immune to poison are unaffected by the spell. Pit Fiend is immune to poison.
7. Acid Fog- Max 12 acid damage per round. Only 2 to a Pit Fiend, which has Acid Resistance 10.
8. Wave of Exhaustion- Good debuff, but not really enough to neutralize the Pit Fiend. It will help soften it up a bit, which it quite helpful, but doesn't neutralize it. Also, allows SR.
9. Geas/Quest- Unholy Aura negates, and has a 10 minute casting time. Great if you have already neutralized the Pit Fiend. Not so great otherwise. As for unwilling help recruitment, must first find said help. Also, the Leadership feat tends to make the ability to recruit help somewhat...common. (I will grant it can be abused, but you are likely to get some...intervention if you attempt to do so) Also, allows SR.
10. Symbol of Death- A commonly misunderstood tactic. Could potentially kill 3000 hp worth of creatures, only if every one of those creatures has HP under 150 (actually says it skips over any creature with upwards of 150 hp). Also, Fortitude and SR negate. (Not to mention you would be spending 10,000 gp in the process, a bit expensive for a one off kill) Further, unless you plan on giving whatever you put the symbol on fire immunity as well, the Pit Fiend will probably destroy it, thereby destroying all the symbols, before you can sufficiently debuff it to get it effected. Allows SR.
11. Energy Drain- Kind of a luck in terms of its debuffing ability. On average it will net you 5 negative levels, but min is 2, and max is 8. In terms of averages, you are probably better off using a maxed empowered enervation, unless you didn't feel like taking five feats just to sap 6 negative levels with said spell. Still, potentially a bit debuff, and good assuming teleportation has been neutralized. Also, allows SR. (Also only subtracts 40 hp from the Pit Fiend's total)
So, strictly speaking, the only spell here that effects the Pit Fiend at all, that doesn't allow SR, is Acid Fog. Everything else allows it, meaning you are going to need to spend three feats to reliably pass it every single time. This isn't a terrible burden, except that even in the case that this does happen, you are only really debuffing it with most of the spells which, while effective, isn't really enough to put them into the save/no save or suck category, and massive debuffing aside, only one spell that will outright neutralize the enemy on the round it is activated. Also, on the note about fireball, so long as the fireball can actually do damage, the Pit Fiend is probably going to keep on using them, because why the hell not. I'm not saying they are going to be doing bucket loads of damage every round, but if you seriously think Acid Fog, which only deals up to 12 damage every round, is worth using at this level, being able to deal 0-30 damage every round to people within a 20ft. burst is going to be worth something. So unless the party, and everything you own, is completely invulnerable to fire, can you think of a reason why the Pit Fiend would
not employ it every round?