Not that difficult, really. You can check my cheese guide for it! I am not a hardcore D&D fan, familiar with rules/rulesets/dices, yet my playthroughs have been a breeze up to now
Note I didn't use the word "difficult". Baldur's Gate is a game that can be very punishing, but there's always
an easy way of doing things, provided you can figure it out.
Even Demogorgon and the Demi-Liches, were nothing more than a few seconds of combat.
Demiliches are a perfect example of Baldur's Gate in a nutshell. They have exactly two attacks, both of which are instant-kill abilities. If you have the right counter-measures you can be completely immune to these attacks and can walk all over them (berserker, anyone?), if you don't they'll just insta-kill your party.
The only "hard" part about demiliches is that you need a +4 or better weapon to harm them. If you tackle the Kangaxx quest early enough you won't have one yet.
Kangaxx early is very easy. Exploit the endless xp glitch. Have a cleric created and maxed, or even better, like me, fighter dual cleric. Kangaxx can turn to dust by being next to him with turn undead (or a sunray/false dawn, because you have some seconds before he starts his instant kills). Next demilich is in Watcher's Keep. You can do watcher's keep in ToB, do not rush to tackle it in 2. When you meet it, you have both spell protections and a nice, +5 or +6 weapon. Fighter mage killed it alone, without even simulacrum/summons tactics. But you have to be cheesy and have maxed xp.
Melissan... I beat her today. The 3 pantaloons (golden, bronze, silver), when turned into the right npc, will result in some crazy items. Target Melissan alone with Sarevok, 2 characters of your creation and your main wearing the robo suit/launcher, and she falls like a cow. Ignore her summoned creatures. Real problem is the fallen solar in there. Nothing else.