Combined with the fact that Mind Flayer stats are abysmal, they'll rarely land a hit on someone with decent AC. And even if they do, just watch your INT and either pop potions of genius or mind focusing or just pull them back. Chaotic Commands (which is cleric-only, by the way), Greenstone Amulets, potions of Magic Protection (which gives you autosave on all spells and spell-like effects), and brine potions negate their psionic effects. Barring that, Animate Dead summons work well (undead are immune to psionics and the skeleton warrior you get at lvl 15+ cleaves right through em), and Mordenkainen's Sword works perfectly well, since it's immune to physical damage and the INT drain. Just blockade the doorway with your frontliners or summons and then just bombard them with arrows, bolts, spells, whatever. You will never encounter mind flayers without the ability to create a chokepoint with the single exception of the Ust Natha quest chain - and there, you can just pre-position everyone out of range of them until after they show up.
With a little forethought, the entirety of BG2 is laughably easy. The Tactics2 mod makes things a helluva lot more enjoyable.
Mages follow a protect-buff-attack AI script and start out buffed with most common buffs (Stoneskin, Pro-Fire/Elec/Energy/Elements, Melf's Minute Meteors, etc) so that you can't cheese your way along with a thief (most common use being to just backstab them for an instakill; thieves with decent STR can quite easily do 80+ in a single hit.) Liches start with said buffs plus a permanent pit fiend pet. Pit fiends (and all demons) are buffed to be more in line with their PnP versions - 6 attacks a round, vorpal hit, poison-and-disease bite, improved invisibility whenever they want, fireball every now and then, and the ability to Gate in another pit fiend once (who can then Gate in another, who can Gate in another...) They still retain their immunity to +2 and below, high magic resistance, and high AC.
All undead (excepting liches and a couple other notably powerful varieties) get minor buffs, to make them a little more threatening; minor shadows get Spook, major shadows get Hold Person or Greater Command, etc. Powerful undead like liches and Kangaxx WILL notice you beating on them with Pro-Undead scrolls and WILL use dispel magic on you. Pro-Magic still works, but those are rare, and now you'll draw the attention of that pit fiend since Pro-Evil is gone...
Sahuagin City is totally redone. All Sahuagin (except the specific named ones) are now Archers, at least Lv.12, using paralytic bolts (1d10, save vs. spell or be stunned 4 rounds) or +3 spears in melee. When you kill them, they resurrect as skeleton warriors. The priestess that carries the cloak of mirroring can summon a minor avatar of sekolah, who can eat characters whole (vorpal attack, permanent death.) She also likes to summon glorified frost salamanders if fighters get close. In short, Sahuagin City becomes EXTREMELY FUCKING HARD. But it's optional.
Major bosses and encounters get a huge buff. Bodhi is actually hard now; good thing, since she's worth 91k XP - more than anything else in the game, except Demogorgon (who is effectively worth 600k.) Dragons will take exception to you summoning monsters or setting traps in their sight range, and use their wing buffet and breath attacks much more frequently - they're still too easy, though. Mind flayers get several new abilities to prevent cheese (they will dimension door to the rest of your party if you try to blockade them, they get nasty knockback attacks, Emotion, and improved stats.)
In both versions of BG2, Breach is the most useful arcane spell in the game (I'd say the un-nerfed Conjure Animals is the best divine; those bearweres are sick.) It will kill Pro-Magic Weapons and any level of Mantle, as well as Stoneskin. Unless they're protected by Spell Trap (9th level spell, usually only see liches use it frequently), which you can counter with Ruby Ray of Reversal, among other things. You can get by pretty well without any arcane spellcasters for the first half of the game - I've played games where Imoen was my only wizard, and I did most of the quests in chapter 2, before rescuing her. Pro-Magic Weapons and the Mantle spell only last four rounds - just go hide or tank the damage till they expire. If they don't require magic weapons to hit (like liches), just go low-tech. Stoneskin can be dispelled by Dispel Magic, or you can just beat it down.
Death Spell or Death Fog will instantly kill any summoned enemies in range - use it if you're getting mobbed by summons.
Beholders are worthless with the Shield of Balduran. It's pretty cheap - about 14k with a decent reputation and good CHA. Tactics makes them less stupid about it, though; they'll just bite you instead of fry themselves, or ignore the shield bearer and fry everyone else instead. They've got surprisingly good attack values, and their bites carry powerful poisons, Hold Person, and other assorted nastiness. A single well-buffed warrior can STILL clear out entire hives, but it's not quite so easy.
Unimproved BG2 is stupidly easy if you're willing to put a little thought into your spell selection and plan of attack