Protect Yourself From Magic
A good strategy to kill mind flayers is the spell animate dead, they are immune to mindblasts and the Skeleton nights you get at level 15 are highly resistant to magic. The normal skeletons will do the trick also if you have enough of them :)

Submitted By: Vidar

Summon The Undead
Undead minions are the key to defeating Mind Flayers. I was able to keep three Flayers trapped in a doorway with four skeletons blocking their way in. The Flayers uncorked all their whoopass mind spells on the skeletons to no effect, while I laid back and cut them to ribbons with ranged weapons. I had fought them for hours before figuring this handy trick out.

Also, the Flayers attack characters who are frozen first, so if you have some summoned creature in the front and he gets frozen, the Flayers will (usually) go after it first rather than even defend itself from attacks made by your main characters.

Submitted By: DJ Swinger

Chaotic Commands
A simple way to defeat mindflayers is to use the spell Chaotic Commands. Walk in there with your best fighters and cut them to shreds in a matter of seconds, simple and resourceful.

Since Chaotic Commands is a Fifth level spell it is very handy to have Wondrous Recall memorized. For the weaker party members I would advise to have some summoing spells handy and then take out the flayers from a distance.

Submitted By: Baldvin

Greenstone Amulet
Mind flayers are made much easier by the Greenstone Amulet. You can pick up the amulet in the beholder dungeon. It has 15 charges, and although it says it does not confer complete complete immunity to psionics, I cleared out the mindflayer dungeon without it failing once. Just give it to your best warrior and get his armour class as low as possible. With Drow full plate+5 and a total AC of about -14 they will require a 20 to hit you, so you should be reasonably safe. Also, always take out the Ulitharids first, as they only seem to need to hit you twice, and have a better THAC0. If they have Umber Hulks with them, fire a cloudkill from out of sight and run.

Submitted By: Crovax (from a post by Jursementastile)

Haste & Cloudkill
First, cast "Haste" on your party members and have one of your party members hold the "Wand of Cloudkill". Before reaching the Mind Flayers, hold your party and control the one with "wand" to move forward and firing at least 3 shots of the "wand" just beyond the door. Upon doing so...immediately rush back and close the door behind you. The psionic blast may hit you even when you close the door, but there is no way they can get through the door to hurt you. In the meantime, mind flayers will be gathering at the door waiting for you to open it. And at the same time, watch, as they gather damage from the cloudkill spell, til their death.

Submitted By: Radiostar

Invisible Stalkers
This is the easiest way to defeat Mind Flayers, Ulithid and Umber Hulks! Just summon a bunch of Invisible Stalkers and then sit back and watch them finish the job. I always summon 3 to 5 of them and let them do the job!

Invisible Stalkers are immune to all kinds of hold, charm, confusion and domination spells. And they cannot be one-hit killed by those annoying Mind Flayers. With these bad boys under your command, you don't have to bother using Chaotic Commands or summoning skeletons. :) Oh by the way... Invisible Stalkers also attack faster than other summoned monsters if I'm not mistaken.

Submitted By: Mitch007

Brain Suckers
A tip for fighting those damned Mind Flayers. Chaotic Commands certainly helps, but they continue to drain intelligence when they hit which can quickly prove fatal. Low-intelligence characters like Keldorn don't fare well at all in the Illithid city. I got into the habit of bringing Potions of Genius and Mind Focusing (finally a use for them!)...the added INT makes meleeing the Flayers much less likely to result in brain-suckage.

Submitted By: Dave

Backstab and Run
I found out after a while that backstabbing and the boots of speed is the key to success when you battle mindflayers. Though you must also use some protection against confusion and stun. I used a half-orc fighter/thief with the boots of speed, and the green stone amulet (which I got from a Mordenkainen's Sword (just cast the spell with your mage and then pick pocket the sword)). When you're ready just run into the room (hiding!) and place yourself behind a mind flayer and backstab the sucker! He will probably be gibbed or get massive damage. All mind flayers in the room will cast their spells but you'll be immune. Then you must run out of their sight and go inte hiding again. Repeat until all are dead :-)

Submitted By: JoDa

Cloudkill Tactics & More
Cloudkill their pet Hulks and godspeed your monsters ... just Aerie and Nalia (or Jaheira) can easily finish those mindsuckers.

It's no use if you sic undead or elementals on the Flayers just to have them trampled by the Hulks. As it is well known, Umber Hulks die quickly when exposed to Cloudkill.

First, you'll have to lure the brutes into your poison. Summon some small beasts like wolves or dogs or spiders (they move faster than bears). It would help if you cast strength and haste on your little beasties as well. Now open the door (the door of an area with more than one Mind Flayer) and tell them to move to the center of the room (out of your range of sight) and close the door. You'll see battle text pouring into your dialog window. While the Hulks and Flayers are busy fighting, have a hasted mage quickly open the door and cast Cloudkill (or use a Cloudkill wand) slightly beyond the door and shut it again (make good use of the doors you're provided, I say).

The Hulks will make quick work of your beasties and will start to advance towards your door. Remember to position your party far from the door as psionic blasts may penetrate through. All you have to do now is wait for the Hulks to wilt as they stupidly pound on the door. The smarter Mind Flayers stay away from the poison, at most getting "Barely Injured".

While waiting, have your mage/druid/cleric summon 4 or 5 skeletons. It really makes a huge difference if you haste, luck, strengthen and bless your skeletons (yes, always bless your monsters). When the Cloudkill wears off, quickly open the door and send your mini army of bones through. I had all my party members positioned far away from the battle except Aerie, who stood in an extreme corner and continuously summoned undead as the Flayers killed them.

If 2 or 3 Mind Flayers still live and you've run out of skeletons to call on, I guarantee they'll all be "Near Death" and Aerie could rip them to shreds herself.

NOTE: never (ever) stand anyone (even someone with a ranged weapon) in the direct view of a Mind Flayer.

Submitted By: Fatale

Filet the Flayers with Mordenkainen's Sword
I just spent some time in the Underdark and found a neat way to carve some Mind Flayers. Mordenkainen's Sword is resistant to their attack, so send two down the corridor and they should clear the room before they expire, otherwise just repeat. Coincidentally they will also beat the Umber Hulks that are with them fairly easily.

Submitted By: Rich

Chaotic Invisibility
One mage with the Staff of the Magi and some patience can kill Mind Flayers. Cast chaotic commands on your mage or use the greenstone amulet. For some ungodly reason, the Mind Flayers cannot see invisible, nor do they seem to be able to cast any spells to allow them to do so. My mage, with chaotic commands, (I used Nalia, believe it or not), would walk in, fire some arrows, and then equip the staff and go invisible. Get back into position, wash - rinse - repeat. If you don't like using up arrows, you can just pound on them with the staff and when they drain her intelligence, go invisible until it wears off. Needless to say, turn AI off while doing this so that you can stay invisible as long as you want and her script won't kick in...

Submitted By: Laurelei

Get Your Death Spell Ready
This strategy is very simple, although you will need at least one mage. Just use Death Spell, a lvl 6 mage spell. The spell kills Mind Flayers, and the best part is that it will kill many with just one spell. I have used this in the Underdark when I had to rescue the Matron Mother's daughter, and in the Mind Flayer lair in the Underdark. When I saved the daughter, I casted 1 Death spell which each of my 2 mages, and they killed all of the Mind Flayers and the other monsters before they even made any attacks.

Submitted By: Dalamar

Backstab, Traps, and Chaotic Commands
What I do against Illithids is hide my whole party behind a door, give boots of speed to the thief and hide him, set a few traps, and finally cast Chaotic Commands on him.

Get behind a mind flayer, backstab him for around 40-80 dmg, then run away (you are immune to their mind blast but their brain suck can still waste you in 1 hit). When you arrive near your traps, hide again. If any follow you, they will either be killed or seriously hurt by the traps, then go back and backstab again, repeat the process until they are all dead.

Submitted By: Orogrog

Mass Invisibility and Invisibility 10' Radius
Mind flayers have no way to detect invisible creatures, so using mass invisibility or invisibility 10' radius, you can walk in, position your characters right next to each mind flayer, and hack them to death before they can do a thing. This works better with mass invisibility as your people stay invisible when you attack, giving them some bonuses that make them harder to hit. This can also be used in conjunction with most other tactics, just make sure you summon your monsters and cast your person targetted buffing spells before you go invisible. I cleared out the mind flayer dungeons in the sewers using my party (Kensai/Mage, Minsc, Keldorn, Jahiera, Aerie, Jan Jansen) plus a couple of skeletons and a fire elemental simply by casting invisibility 10' radius before each room.

Submitted By: Deathray