I play on core and everyone is single classed (Except Regil who starts with 2 classes obviously). Some fights were tricky and required several tries, but I am now in Act 5 and face no problems anymore (no exploits either).
The gear you get later and the Mythic path powers really level up the field. By the time I reached chapters 4-5, I had to upgrade the difficulty because the lich spells were wiping out many of the encounters by himself and the trickster with persuasion 2 was making almost all the enemies frozen with panic besides the tougher ones. I had some troubles with the Aion but when you start getting the good spells and heighten the others things get even.
There are spells that are useful the entire game if you heighten them and some of them ignore SR, like blink (ignores true sight), grease, glitterdust, the inflict wounds/mass inflict wounds/slay living/harm (if you use the range metamagic they become ranged touch attacks) etc.
I thought at this time they would have fixed the cave fangs exploit in WOTR, in PF the mod added an option to allow to set only one stalagmite trap per turn, but I suppose we have to pick a mod to do that again =D Like with a button to toggle on/off the ai and stealth mod. On the mod page, the modder just wrote "Well, here we are again". Luckily these mods seem to be easily ported to WOTR.
