Is there an efficient way to defend from illithid's psionic blast except with an high will save? Item, spell...?
Be immune to being stunned.
The easiest way to do that is using any spell that changes your type into undead, elemental, construct, plant or ooze, or give you immunity to stunning right away.
Favor of the martyr and elemental body, both from Spell Compendium, do the work just fine. Favor of the martyr gets special mention for having a lot of other immunities, including the rare and coveted daze immunity.
Ernir's Lists of Necessary Magic Items has a list of helpful items for every situation, including giving immunities to stun.
- Third Eye Clarity (MIC). 3000GP, face slot. Once per day, negate the Stun as an immediate action. Can negate other nasty stuff too. Great pick.
- Talisman of Undying Fortitude (MIC). 8000GP, held. Swift action activation, gives you lots of undead immunities for 3 rounds, including stun.
- Banner of the Storm's Eye (MIC). 15000GP, held or shoulders slot. Negates stunning and other nasty stuff for you and your allies. Top pick.
- Wakeful Mind graft (FoE). 14000GP, slotless. Other minor benefits, including no longer needing to sleep. Also makes you slightly less healthy and less knowledgeable, but that's the price you pay for having robot components embedded in your head.
The psionic blast of a mind flayer (MM 187-9) is spell-like, rendering it nearly immune to counterspelling but not disruption (MM 315), so a mind flayer might not expect the skilled archer's readied action ("When he starts staring off into space, I open his esophagus with an arrow!"--assuming that the DM permits characters to know when creatures use spell-like abilities and that mind flayers have esophaguses). The psionic blast isn't mind-affecting, so you're not helped by gaining immunity to mind-affecting abilities (e.g. the spell mind blank [abjur] (PH 253)).
Immunity to Mind Blast
The only thing I've found that specifically protects against an illithid's mind blast is the head-slot item neuroshield helm (Dragon #291 50) (23,000 gp; 3 lbs.), that says, "If the wearer... is mind blasted by an illithid, [the wearer] is completely unaffected...." The helm stops 3 mind blasts before the wearer must discharge a mind blast himself (an ability granted by the helm) to be immune to 3 more mind blasts--and so on. It's expensive, but unless surrounded an inquisition of mind flayers it should do exactly what you want.
Get Out of the Way!
The psionic blast is a cone, so exiting the area--however briefly--might be a thing.
- The shoulders slot item shadow cloak (DrU 101) (5,500 gp; 1 lb.) as an immediate action 3/day grants the wearer the ability to teleport up to 10 ft., which might be enough to get out of the way if you're at the edge of the 60 ft. cone.
- The 3rd-level Sor/Wiz spell permeable form [trans] (LoM 129) as an immediate action for 1 round grants the caster the benefits of the incorporeal subtype (MM 310-1), giving the mind flayer's psionic blast spell-like ability only a 50% chance of affecting the caster.
Gain Immunity to Stun
Many methods exist to avoid becoming stunned; the best of these items Zachiel's mentioned. (The face slot item third eye clarity (MIC 141) (3,000 gp; 0 lbs.) is a personal favorite of my single-appearance NPCs.)
The best of these spells Zachiel's also mentioned, but talk to the DM about freedom of movement [abjur] (PH 233) and like effects (e.g. the granted power of the travel Domain, the activated effect of the spell heart of water (CM 107)) as the rules are unclear about the limits of the spell freedom of movement.
The 5th-level Drd spell plant body [trans] (SpC 159), the 6th-level Clr and Sor/Wiz spell stone body [trans] (SpC 207-8), the 8th-level Sor/Wiz spell iron body [trans] (PH 245), and the 8th-level Clr and Sor/Wiz spell veil of undeath [necro] (SpC 229) also grant immunity to stunning, but the 7th-level Sor/Wiz spell elemental body [trans] (SpC 78-9)--with its duration of 1 hour per level--is the real winner.
I'd be remiss if I didn't mention the 3rd-level Clr spell mass resurgence [abjur] (SpC 174), which grants the caster's level in creatures a new saving throw versus an ongoing effect. If the party totes used wands of mass resurgence with a couple of charges each and the Use Magic Device skill ranks to employ them, the party will be much safer in general, not just versus mind flayers.
...Or Take a Feat
- The feat Quick Recovery (LoM 181) grants a dazed or stunned creature a new saving throw to end the effect when the creature begins its turn dazed or stunned. This is probably what the game expects you to take if you're regularly trying to mundanely sword mind flayers. The lack of prerequisites makes it available immediately.
- The feat Mark of the Dauntless (Dra 142) grants the creature immunity to dazing and stunning and the ability to touch a creature as a standard action to remove those conditions. It's specific to Eberron, requiring a true dragonmark as a prerequisite.
- The feat Disrupting Spell (FC2 83) increases a spell's level by 2 but lowers by 2 a creature affected by the spell's save DCs versus its spell-like abilities. However, the description is careful to eliminate further tricks you want to pull with the feat (e.g. multiple disrupting spells on the same target, 0th-level spells), which is sad; this isn't recommended unless you're routinely facing inquisitions or--and I hope you brought an army o' plants and zombies!--whole cults of mind flayers.
Spell immunity protects you from one spell or spell-like ability of level 4th or lower (Mind-Blast is the equivalent of a level 4 spell). You must define that spell upon casting. This would make you immune against a Mind Blast for 10min/lvl.
Spell resistance gives you SR 12 + Caster Level against a regular Mind Flayer's d20 + 8 Caster Level Check.
Protection from Spells gives you a +8 bonus on saving throws against spells and spell-like abilites. This would not only give you quite a bonus against the DC 17 Mind Blast, but also against the Mind Flayer's psionic abilities.
These spells are mentioned in Lords of Madness p.175, where it also says that Mind Blast is [mind affecting], which is an error, since the Monster Manual does not state that.