I was near the end of the game when I switched to another game. I will get back to it later.
I played a pure Mage, not putting a single point in any other attribute other than INT and and any skills other than Casting and the other important Mage one that raises your mana regeneration. So I had access to the highest spells, hit for fantastic damage, with plenty of mana that kept regenerating fast. As a side-effect enemy spells do nothing at all to you as you're practically immune. For defense (not that I needed defense when everybody died before they could attack), I could run away fast, fly or turn invisible. For fun, I had enough mana to even enter a room invisible and Mass Incinerate everyone killing them without retaliation. Of course, it was much more efficient to queue up multiple fireballs in advance and unleash them as soon as you see an enemy.
I blew up everything including the Ylside and the demon without the slightest difficulty. Liches were a joke as I could have killed them by punching them in the face, as their spells were useless against my resistance. I know there's a dragon coming up, but I haven't faced that yet.

Just thought I'd add my own tip to this thread for anyone else who's hitting this point in the game. I'm a full mage, but I'm certainly not wiping them out easily like another poster here suggested I should be able to do. I had tried 2 fireballs plus Harm, but they didn't die from those before slicing me up in the many attempts I tried for that.

I also tried sneaking past, but invisibility didn't help. They can apparently tell *something's* there, because even if they don't attack when you're invisible, they follow you around until your invisibility runs out, and then of course kill you.

I'm also one of those unfortunates who can't go in through the back door, even though I left through it before ("Strange...it should work." Yeah, I agree.)

I was ready to give up the game entirely if those options didn't work, but I ended up paging through my spell book for more possibilities, and tried out this combination, which surprised me by killing it.

First, you might want to have Harm running, just in case, and also keep a mana potion and/or an offensive scroll on hand in case this doesn't take them out completely. Pre-cast Fireball, Paralyse, and Incinerate. Start by softening him up with a fireball. Then, when he's running toward you, cast Paralyze. You'll need to double-click on him once he gets close enough, and then it'll take effect. While he's paralysed, cast Incinerate (again, you'll have to double-click on him to do it.) All the while, your Harm spell should also be damaging him. Hopefully, the Incinerate spell should kill him when it explodes (will take a few seconds, and he'll be able to start moving again before that happens).

You could also cast Paralyse again to buy yourself some more time to hack away at him, if you can do it quickly, but you'll probably have to drink a mana potion before that, or use a scroll.

This may not be applicable to most characters, but as it hasn't been mentioned yet I'll note it here:

I recently played through a super-thief character that only raised Dex, thief skills, and projectile weapons. So: crap HP, weak armor, and sub-20 casting (rats survive my fireballs). The magic of bows is that they get a significant damage bonus with range, so at high skill you can one-shot an Ylside if you hit him at max vision range. In fact, there are a number of areas long enough to shoot an enemy from well beyond visual range, and you can one-shot an Ylside or a Lich this way with projectile skill around 60, possibly less. The long entrance hall should qualify [as do sections of the Crypt], and if you don't get a kill, you'll have time for a strong second hit because they only slowly walk toward you if you're far enough away / hiding when you hit. Works even better with Inut's Bow, of course, but the normal one is sufficient and far easier to aim (the bottom center dot in the H-like symbol just above your hand).

This approach works best if you can enter from the back, since you'll have a nice long shot at every one, but it should work for the first and third Ylside coming from the front as well.

And overall, this is just part of the whole "Linear Warriors, Quadratic Mages" issue common to RPGs - warriors usually have an easy early game but eventually get outclassed, while mages struggle to survive at first and then become tiny gods. It's hard to avoid in part because a pure-melee fighter has few tactical options, so the only way to make an enemy a credible threat is to make them really dangerous.

The potion-burning solution is probably the "right way" to play a late-game fighter, but look out - potions set off a heal-spell-like effect, so they may be healing nearby enemies as well: try to back away as you drink them! If you were staying in contact, you may have been lengthening the fight quite a bit.

It's been a while since I played this but for me it was basically all of the above. I.E. be a complete whore with scrolls, hit and run, everything haha. I think I was able to get some cheap hits by going up the stairs at the near end of the first hallway and jumping off the bridge in the alcove over the entrance all while firing arrows like a madman. Or maybe they figured me out :D haha I don't quite remember. But yeah, go balls out! This is definitely the hardest, most ridiculous part of the game imo. Use any and all paralyze scrolls you can get, and use them wisely

Diggin up this old thread, cause I'm in the same situation, playing a pure fighter with only 12 points in casting. By far the easiest way to deal with Ylsides for me is this: I precast 3x harm and live-cast paralyze when approaching an Ylside. Once he is paralyzed, I spam 3x the precast harm, cause the damage actually stacks. This way, the Ylside goes down in a matter of seconds. Being close up, I still need 3 healing potions/Ylside to survive their insane damage output.