beholders? no problem - cloak of reflection thingy and ring of free action, and my main character could take them out easily
dragons? i hate 'em, but hey - they're meant to be barstewards, so i can live with that
golems - i never met many, and was supremely lucky whenever i did (windspear dungeon with the mob of them near the well? they got stuck, and the adamantium one nuked the others with that area effect spell they have
mindflayers? the first time i met one of those buggers i took a beating, but when got to the lair i had levelled up loads, and took em all out with my main dude and summonings
however, umber hulks in the early part of the game annoy the hell outta me! they're tough, strong, fast, and use physical attacks. thus, there is not much you can do to prepare other than slap on a chant or protection from evil or whatever and hope...