Mind Flayers are not that tough, depending on the class of course. I have gone thru them with just one char 2 times - berserker and cavalier. All you really need to keep in check with them is your INT. Melee wise they are pathetic. So, get the Boots of Speed (quest, doable easily), also drink the Oil of Speed to get the top notch speed, stack the char with the Potion of Genius and-that-other-one (forgot the name) which adds +3 to INT and DEX. These stack. And then waltz in. Keep whacking them and checking your INT. If it gets close to 10, drink another potion, even both, that will boost the INT again. Done.
Adamantite Golems are pests, but subject to MMM. Shoot, cast, shoot, if run out of spells, rest, cast and shoot again. Keep in mind though these smaller golems, who do fit thru doorways and such. Clay Golems are pests, they like to Haste themselves..
Greater Werewolves or Greater Wolfweres are pests indeed, the regen.. They need to get a lot of damage and at once. Yoshi's snares do them in nicely, 5 snares killed one outright and almost killed the second. They still put up a good fight, as they regenned fast. So these are definately not easy. But, you don't meet them so much anyway so it's not a constant issue.
Beholder kin are not so bad, once you have the gear. Cloak of Mirroring is definately good thing, though it will be available much later. I personally use the cloak on the main char, as he's usually in front, gets spotted first and nuked as well first. Casters in my party do not get that cloak, they should even BE so close to the enemy that they would be spotted and nuked first. As for the Shield Of Balduran, then it's not cheesy. It's a useful in-game item, works specially against one type of monsters and also has a slight penalty. In my eyes it's far from being a "cheesy item". Without both of these though it will be tricky to fight the beholder kin.
Vampires are pests only as long as they can drain your levels. Once you get "past" that, be it a spell or the protective item, they are easy.
High lvl caster mobs are tad pesky, and only for 1 reason - Imprisonment. Other then that, they are not such a threat. Chop off their fingers and no more finger fiddling to get a spell off. Other then that, I don't mess with them. Males in dresses, bah. Real men wear platemail