- At close range - release a green energy ball which causes paralysis, followed by a tentacle stab for heavy damage.
- At long range - cast a powerful Soul Ray spell
- Casts a sonic boom for heavy damage. This is not proximity dependant, but does tend to follow a tentacle stab.
They're easy to spot as they glow and carry a distinctive bell. This bell seems to alert the other guards, who will attack if they are nearby. These guys tend to kill new players with magic. If you make a direct frontal attack, they will stun you with their magic. Only a shield that blocks magical attacks will be effective. So, how to beat them? Fighting magic with magic works fairly well; if they rush you, just keep casting from behind them and they'll drop quick enough (fire magic really messes them up). You can also equip the Thief's Ring and stab them in the back for a critical strike. Try using a bow at long range as well, just be ready to dodge their magic spells.
If you're a melee player, your best bet is to wear the Thief's Ring and hide behind the final pillar in their patrol route. As soon as they turn round do a strong attack in their back and then get a fast attack in too. As long as you hit them before they start casting they will be staggered, and you can keep attacking as long as your stamina holds out. If they start casting before you hit them, roll and pray. (You can also just run up behind them when they in a narrow corridor on their patrol route, you can just just stand behind them without them being able to turn, this can give you an opportunity to position yourself to back stab and think of how you want to deal with them on the ground.). Another method of taking them down quickly is to hold a heavy weapon with magic attack power in two hands and use your strong attack (R2) on them, this can take them down in two hits.
In world 3-3, you'll notice the Mind Flayers are facing forward and due to the narrow staircase you can't backstab them. The best way here is to get close enough so they stop casting their Soul Ray, and start casting Paralysis attack. You must stand against the inner side of the wall, and keep enough steps distance. Even though their attack will hit the wall and miss, they'll still run towards you. Hit them hard (R2) once, then quickly roll away to evade their barrier. Preferably, if you have a large hammer (Meat Cleaver, Bramd, ..), a large axe or Dragon Bone Smasher then double-hand it, roll towards them and press R1 for almost-instant (and huge) damage, then quickly roll back. Normally, after rolling away, you're close enough for them to do their paralysis attack.. but it will hit the wall again, and they'll run towards you. Repeat this process untill they finally die. If you have a weapon with knockdown ability, that can also be handy as you should constantly hit them down when they try to get up. The Eternal Warrior's Ring helps greatly in this process.
Another strategy, partially mentioned above, is to get to the range where the flayers stop casting soul ray and try to paralyze you. Once they cast their paralysis spell, simply walk backward (holding backward while locked on), and the paralysis spell should dissipate without hitting you, because of it's short range. At that point, the flayer will still charge at you, thinking it's spell actually hit. Start a power attack as it is running at you, then keep the combo going. If you wait too long to start your attack, the flayer will not stagger and will cast another spell and kill you. If you cant kill the flayer in one combo, rinse and repeat as needed. Also works on BP's. Tested to NG++.
Note: If you are experienced with their method of attack, you can try to bait them into using their paralyzing attack. Whether it hits or not, they will rush up to you. Dodge the attack (back out of its range after they spot you) and take the opportunity to bash them in the face. Beware that they may sometimes try it again, or soul ray you when you back up. But if you can recognize their attacks, you can melee them without getting hit, or wearing the Thief's Ring.
Yet another strategy relies on a quirk in their sight / detection method. If you sprint (only tested with < 50% equip burden) towards them until you are close enough to hug them, they won't see you. You can stand next to them for as long as you like without them noticing you, however sometimes they do their area of effect attack even without seeing you, so watch out for that. Do this when they have their back turned to your approach path and get a quick and easy backstab in.
Similar to melee in that you'll probably want a Thief's Ring and to be behind them. Just pop out an hit them at a distance in the back. Before they begin casting, the arrows should stun them for a moment, and combined with their slow turning gives plenty of time to kill them. Depending on the range, you probably will not be in much danger of their paralyzing attack.
Flame Toss should finish a Flayer off in 2-3 hits if you have a decent magic stat (2 hits with Magic 23, first hit takes about 2/3rds of its health). Here, you want to use the Thief's Ring to observe their path at your leisure. The level is designed such that there are almost always places to hide, whether around a corner, through a door, or down a hall. Use this time to lock on and time your opening strike - the first hit will generally incapacitate them long enough for you to finish them off without retaliation. Consider using the Cling Ring as a safety net with low HP or magic defense. They are unlikely to have time for more than one spell even if you botch the timing, but that one can be painful if you're not built to soak much damage.
If you have learned the Anti Magic Field miracle, cast it around you and fight them in melee. They are unable to cast any magic, and thus are rendered helpless.
Note: They also come in a dreaded Black Phantom variety. This super-charged form fights the same, but does more damage. It is also more likely to drop an Old Spice or two when killed.
Fun Fact: Mind Flayers are based off a concept of a Dungeons & Dragons creature of the same name. The stun magic resembles the D&D version's psychic shock that stuns prey so that they can feast on their brains. Also they bear resemblance to the infamous Cthulu from its self title fantasy games and series.