*SPOILERS*
I'd be grateful for any tips on how to fight the Ylsides and survive. Entering the "evil temple" behind the tavern for the second time (with Krahoz and Zohark acquired), I just get beaten into tiny little bits and stomped on by those super-warriors. I'm basically stuck as I cannot seem to beat them.
I have a fighter-type character, that is, I have concentrated on melee skills and not spell casting nor sneaking (etc.), so my melee skills are about as high as one could have at this point in the game (granted, I haven't pumped ALL my points in melee). Also, I have, IMO, pretty good gear on (fully decked out in plate with the enchanted plate leggings from the rebel hideout) and have so far been able to hold my own vs the enemies the game has thrown at me - e.g. the lich in the crypt nor the snake-women guardians in the temple of illusions (or whatever it was) were not much of a problem for me. However, facing the Ylsides is a *major* pain in the backside. They, more or less, just "steamroll" me no matter what I try to do. Even swearing and shouting insults at them (i.e. my monitor) doesn't seem to help (believe you me, I have tried ;)).
So, any tips on beating them Ylsides mano a mano? How in the nine hells did you beat them?! Do note that my magic skills are in the low 20s (IIRC), so no fancy magic tricks for me (I do have loads of scrolls, but fireballing those buggers doesn't seem slow them down much).
P.S. Just thought of this, but I haven't tried poisoning my blade yet. Hmm, there's an idea...

Unfortunately, the character types aren't very balanced in this area, and fighters have the hardest time versus ylsides. Mages can wipe them out without breaking a sweat: fireball, and if necessary, follow it up with chaos.
One of the devs posted in this thread (beware spoilers) on the JoWood boards, and basically... you don't. You aren't intended to be able to fight them yet. He advises taking off your shiny clanky armour, and sneaking past them, even if your stealth isn't very high.
Failing that, he recommends getting all the mithril equipment...
Other than that, fireball is one of the most effective spells, so having a few precast and tossing one off while you wait for him to get into melee range can only help, even your casting skill isn't very high. Harm is another useful spell that even the most magically inept character should be able to cast -- low mana cost, and helpful in close-quarters fighting.
Poison will help end combat quicker, but by itself won't be enough. It's pretty hit-or-miss as to whether a successful hit will actually poison the target, and if it does, it works pretty slowly. Still, any advantage you can get, you should take.

Fireballs will only seriously trouble the Ylsides when your Casting and Mental are quite high (a high-level mage can kill an Ylside with one or two Fireballs). Harm works well, but the huge level of damage they deal at melee range makes it quite difficult to survive.
Another option is using Invisibility. You can brew this as a potion or collect the runes for it; the potion form wears off in its own time, so the spell is preferable. While invisible you should be able to launch at least one Fireball or whatever before you run out of mana or do something that makes you become visible, and the Ylsides will only be able to dash around madly trying to find you.
I actually gave up on my first character and started over with a less-balanced mage-thief; I was able to easily obliterate Ylsides with a couple of well-placed fireballs each; most didn't even get within melee range despite their super-speed ability, and when they did a fireball to the face was generally sufficient. The downside of a mage build is that you'll have far less health, so when an Ylside closes in you need to be able to finish him off immediately or you'll be dead in a second or two.
The easiest way to deal with the temple revisit is to run in through the secret door in the goblin dungeons on level 2 (the secret door is in the indent just to the right of the stairs you go up to reach the overpass leading to the goblin kingdom). Going in that way will take you right to the meteor room which only has one of those purple-robed priests guarding it. Iserbius's security is rather poor. :) I'm not sure whether the remaining enemies leave after you destroy the meteor, but you may be able to go back there later on and finish off the Ylsides for tons of experience.

What about hit and run tactics??

Another possible solution is to lure the Ylsides into the no-magic zone (the weird flesh room). They can't make it across the platforms there and can't use Speed within that room, so you could shoot them from safety or even try to do hit-and-run combat against them.
I didn't even bother trying. Ylsides can use a brief Speed-like spell, so any time you get too far away they will dash towards you; when you are too close they will simply hack away with their powerful two-handers. Their Speed seems to be as fast or even faster than the one the player can cast. Additionally, doing hit-and-run combat with Speed cast would be much more difficult because the margin of error for getting the hit in and getting out of there is much smaller, and the Ylside can close the gap without too much difficulty. I suppose you could stand around and wait until they run out of mana, but I'm not sure whether this even happens, and even then they are still formidable foes when within melee range.

This doesn't always work. I tried it on my first run, and couldn't open the passage. Am Shaegar just whined, "It should work..."
The problem in the temple is that it is so small, there is very little place to run to.
Other than that, yeah... go invis, toss a fireball, and run like hell before your invis runs out. Wait for your mana to recharge, repeat as necessary. Tedious, but as long as you can find a safe place to hide it should work. Any room that can be locked should do.

I'm not sure, but you probably need to activate it on your first visit before you can access it from the outside. I always go out that way.

That's what I did, and it still didn't work :(

Thanks for the tips!
The thread on the Jowood boards was very helpful - well, at least I got new insight on my plight. Strange how the game, which has so far been very well-designed and balanced (IMO), can be so horridly unbalanced in this instance. That is, mages just nuke the Ylsides (while laughing at them mockingly, no less) and fighters are... well... screwed ten times over. THis one is a design flaw in my book.
As to the fact, that you (as a character) weren't really meant to face the Ylsides yet is very strange quest-wise, as you have to go into the temple (now filled with Ylsides) to advance the main quest, and as a fighter that usually means kicking down the front door and laying waste to the opposition. Sneaking in from the back door - bah, that's what thieves do! ;)
OK, gotta check out this steathy approach. Here's hoping the secret back door works for me (I did leave through it when I last exited the temple).

This is why I quit playing the game, three Ylsides in the same room resulting in instakills for hours isn't very inviting.

Where did you find three? During the temple revisit? The ones in the temple have patrol routes; they will only be together if you rush in before waiting for them to go on with their routes. You can watch from the doorway at an angle until they have gone; as long as their backs are turned they shouldn't notice you there even if you have low Stealth.
Under normal circumstances you will only face one or two at a time.

After I killed the first one in the corridor leading to the hallway with the three doors, two of them came from the two doors and attacked me.
I managed to load/save so that I could get the first one out of my way but the two are still trying to attack me together.

The temple is quite doable even for a warrior if you manage to lure them to that bridge over the pool of goo. They seem to fall down every time they attempt to cross over. When they get up the stairs hit them and retreat backwards over the bridge again. Cheap tactics, i know but it is quite easy to kill them this way for good exp. When they overrun the city however they are much harder to take down, because now they can cast speed.

Well, the Ylsides still hit like a freight train even without Speed/Haste and to get to the pool of goo you'd have to sneak by two Ylsides first (EDIT: unless the back door works for you) and then hope they won't all pile up on you when you fight the third one near the pool. And luring the first Ylside to the pool of goo would be really tricky as he's quite a distance away from it and able to cast Haste on himself all the way to the pool. Seems very dicey, IMO.
Anyways, I tried sneaking in through the back door, but as luck would have it, it was closed for me too. My only option was to go in through the front door. *sigh* Gritting my teeth, I equipped myself with 20 gallons of health potions, wore two rings of protection, poisoned my Ciprian Long Sword of Force, cast Increase Armor and that damaging field spell (Rhaa + Vitae) on myself... and went in. I came, I saw and I was quaffing potions like there was no tomorrow (well, there really was no tomorrow without those potions), but eventually I conquered. Yay me! ;)
But let me tell you, it was hell and I would NOT like to do that again. With a 7th level character, with 45 AC without spells, 117 HP, Melee at 90%, Defense at 70%, I barely made it. Had to quaff 3-5 health potions per Ylside and still end up only barely winning with the HP orb/meter 99% empty. The fights were so terribly lopsided, I wouldn't call them challenging or fun, it was more like hitting your head to a brick wall.
But, in the end, fighting your way into the temple IS doable with a "pure" fighter. You just got to have loads of health potions and lot of patience.

My favourite trick is combining create fire field + create (force) field spells. Works in narrow corridors (so that Ylside cannot get around the force field), like Akbaa temple (good for first 2 Ylsides you meet when coming from the tavern), stairs leading from the outpost to the Arx city, corridor from the castle teleport to the main hall etc. - I cast fire field, immedialy cast force field and simply wait while helpless Ylside dies in flames, drinking mana potions occassionally to keep the fields active, with casting around 30 - 40 I need 2 - 3 potions for one Ylside.