Well, let me start!

I have one paladin, main character imported from TOTSC.

When he killed DEMILICH, he was 13level.
His stats are..

HP:140
AC:-9
THACO:0
WEAPON:Holy Avanger +5
Magic Resistance:81% (50% from avanger, 5% from amulet of resistance (Unseening Eye), 26 % from Jaheira spell 13levelx2%)

I didn't wont to risk ANYTHING with those 19%, so this is the tactics...

All of the party remained at the door,only paladin went to the tomb.
Then when lich shows up, you wait.
He will cast time stop, then some wards, some death spells, which all you should pass.
If you cast or use ANYTHING now, in this phase when he casts timestop, you will lose all powerups.
So wait till he is finished.
Then he will raise his defences, now is the time to act.
Get away little from him, he is REALLY slow.
Drink next potions:
-Speed
-Storm Giant Strength

Then here is comes.
Use most powerfull scroll in the game.
It is Protection from Magic.
There are only 3 in all the game, so use them good.
Now, get close to him, and start pounding.
He will summon gennie, no matter, kill him too.
Keep on bashing, his defences will eventually go down, and you will start to give some serious damage.
When he turns into demilich, keep bashing, he cann't do a SINGLE thing, because you have unbrekable 100% magic resistance (Champion of Khorne-WARHAMMER)
When his defences go down, you will pound about 5-6 hp of damage.
He has 60hp, so it takes about 1 minute of real time to kill it.

That's it.

Variations on a theme.

You will need a warrior for something like this, so if you don't have paladinm, use a warrior/flame tounge +4vs undead combination.
It is pretty much easier with paladin, but without scroll of magic protection, you can experiance some trouble.

did it almost exactly the way you did, only i had 1 scroll of magic protection and everyone else but my stalker was hiding by the door while he stood by the tomb and used the +4 warblade (NOT a weapon or a style he is proficient in)to carve up the demilich. ■■■■ it took a long time as he was all by himself, for a while there i thought my scroll was going to expire and my stalker with it but eventually i wore him down.