Is there an efficient way to defend from illithid's psionic blast except with an high will save? Item, spell...?
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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 cast elemental body, both from the Magic Item Compendium do the work just fine.
http://www.giantitp.com/forums/showthread.php?t=187851 has a list of helpful items for every situation, including giving immunities to stun.
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.
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