Have the mages keep animating the dead, unless all the fighters have totally high level anti-undead stuff, they'll bash uselessly at the stoneskin also kept casting by my mages while the skeletons are hacking away with their swords at the fighters backs.

I've taken out liches and beholders using animate the dead. my high level skeleton warriors pin down the enemy, distracts them, and unless they have to cover a killing ground to get to the enemy, their attacks do a great job of spell interrupting. also I like to imagine the horror of a beholder, lord of all it sees, being attacked by a wave of undead cuisinarts hacking away at you. also, using the project image cheat, I once experimented with 12 skeleton warriros to attack the demilich. didn't work, though.

I don't now , but why only mages
have low HP , but by the mages its depents(spelling) of ther AC , and the distance of where the fighters stand.

Sarevok was pretty much a fighter

The higher the level you get the more the odds favour mages. They keep getting better spells. But are you going magic items or no items? Fighters can get a high magic resistance, cloak of reflection, sword that causes silence, immunity to charm, invisibility potions etc. If you're using items then one runs up invisible with cloak of non detection and Namarra. Another one or two shoot mages with frost arrows or arrows of dispelling then others just beat the crap out them. Stun each wizard one at a time with Celestial fury or drain their levels with Black razor.

Well, although I made that joke up above, my money would have to be on the mages. I mean, as long as both parties knew about the combat before hand, there are just so many possibilities that a Mage party could prepare for. Think about it. They could work together and have 1 mage memorize all of the summoning spells. 1 mage with all the prep spells (cast before combat). Then you have another mage with defensive spells (can we say walls of ice/stone/etc. Not to mention imprisonment or what not. Then you could have a mage with just fire based spells (if the one mage that casts wall can completely surround the fighters) to fireball between the walls and totally destroy the fighters. That still leaves 2 more mages to cast whatever spells they would deem necessary left over. I mean, we are talking about just 6 fighters, right? This still does not include magical items. Yes, if a fighter party had some kind of 100% magical resistance, they could weather a lot of that, but there still is the summoning spells and gate spells that could bring something very difficult to fight. It's the mages by a long shot. At low level, the fighters have the mages skunked though. Also, if there is no preparation before battle, more than likely the fighters would win too.

Well I STILL think 6 fighters!! I haveso much magic resistance and not a single spells can get through me. I killed loads of cowled wizards by just standing there until they waste all their spells!!.

And oh yeah - I fight pit fiends for training so that isn't gonna stop me.

You can't have 6 on 6. You can have 3 on 3 however.
Try having one archer with a stack of dispelling arrows and the Thurigan Bow. Make one wizard slayer, and the third (beserker for resistances probably) who can wear the helm that grants simcrulium. Equip two with the boots of speed. This gives you two beserkers and a wizardslayer charging into melee, one for each mage. The archer will strike at all the mages. However, because you can't pause in multiplayer the archer would have to be extremely deft to fire one arrow at each mage at a time, and he cannot wate time to switch to another stack through inventory, he has to pick the right number of dispelling arrows so that he runs out and goes to biting arrows.
One thing to point out is that you can't summon monsters effectively to fight another player because the other player can probably control those monsters as well. If only the mage party can control them, the fighters would be able to reduce the mage effectiveness by summoning their own monsters through horns and items, and reaching the limit on summoned, depriving the mages of their ability to summon. Use of the horn of blasting may also interrupt casting. A smart fighter player should be able to beat the mages consistently.

I also have another fighter dual wielding the FLAIL OF AGES + 3 and FROSTREAVER + 3, so the extra 1hp of frost damage, acid damage, cold damage and fire damage keeps on interupting the spells!!

And no, stoneskin doesn't help!!

Lower Resistance
Breach
Tanor transformation
Greater mantle
spells fighters can't stop
if you are at 160 magic resistance that is about level 50 or something
inmagine the power of a level 50 mage

Coweld wizard are stupid and don't know how to use spells