The contest is closed. I wish to thank all players who participated and sent answers. All of the answers I received were basically correct. The two best answers were from HTRT and lambada. I will quote their answers to the contest first; then I will summarize the essential points and at the end I will announce the winner.
HTRT's answer:
QUOTE
I would say first and foremost that the three grave liches do not seem overly aggressive in their tendacy to debuff the necro..but more than that, I think that even if they did, it wouldn't help them that much, because improved alacrity (and a very fast casting time) are actually your best weapon and advantage over them. They have decent casting speed but cannot get their aura to be cleansed, so even if they could breach you, you could still rebuff under alacrity before they would have the time to take another action on the matter. The effect of improved alacrity being superior in this fight is quickly felt by the player when you get disrupted by larloch's disrupter level 5 spell. You realize at that moment how much you rely (and should rely) on improved alacrity in this fight. Larloch's disrupter being a necromancy spell, it gets pretty natural to use it instead of SI:abjuration. Moreover, with SI:abjuration you will only be protected against larloch from your spell turning, which is the very first layout of defense that will be removed by a ruby ray
lambada's answer:
QUOTE
It's all about casting times. SI:Necromancy keeps you safe from dreaded Larloch's interrupter. For as long you keep Improved Alacrity active (and it can't be dispeled) most important protections - protection from Fire, PFMW and Stoneskin can (with improved casting times - Forknowledge, Amulet and Memory of Apprenti) be cast instantly. Therefore, you are safe for the duration of Alacrity, but only if not under the effect of Larloch's Interrupter. Second, SI:Necromancy provides protection from ADHW. Grave Liches will cast it on the Old One. If your protagonist is in AOE, he will be vulnerable to PW spells or even dead. Protection from Magic Energy has a casting time of 6, which is too long to be used in the middle of battle. It's (PFME) a level 6 spell. These slots are very valuable for PFMW and Globe of Invulnerability castings, since casting it from scrolls messes up Alacrity. It also gets removed via Breach/Remove magic, unlike SI. Spell Immunity has a casting time of 5. Not much faster (still a bit above ista-cast) but much safer. Grave Liches have less means to remove spell protections (Turning, GOI and SI, none of which are affected by Remove magic) than combat protections which are vulnerable to both Remove magic and Breach spells.
Summary:
1- The main enemy spell you need to worry about is Larloch's Interrupter, which happens to be a necromantic spell. SI: Necromancy protects you against it.
2- Ruby Ray of Reversal always removes higher level protections first. Hence, Spell Turning alone is not a reliable protection against Larloch's Interrupter. If you don't use SI: Necromancy and merely rely on Spell Turning, it is very unlikely that you can complete even your first Improved Alacrity spell (unless you steep to the level of a cheater or use some very cheap and questionable methods).
3- You don't need to worry about being Breached, because as long as you are under the effect of Improved Alacrity (and refresh it each time before it expires), you can refresh your combat protections in no time whenever you get breached. Hence, you don't really need SI:abjuration.
I have yet to see even one single example of anyone having won this fight without using SI:Necromancy in a legitimate and clean way without resorting to either cheating or utterly cheesy methods.
On a related note, one known cheat (pausing the game and starting Improved Alacrity before the Old One's dialogue) has been completely blocked in IA v7.
Both HTRT and lambada's answers were really good ones, but since we are going to have only one winner, I announce HTRT as the winner, because his answer included the important point about Ruby Ray of Reversal removing higher level protections first (the reason Spell Turning alone is not a reliable protection against Larloch's Interrupter spell).
HTRT will be awarded +1 Tactical Reputation point. Congratulations!