There are two things people find challenging about the lich tower section:
The tower – there is a note left next to a corpse which gives the hint that some stairs are cursed. they may look like they go DOWN when they will actually lead further up. Which requires you to warp your mind and when entering a certain level and still standing on the stairs you came up, press DOWN and then you advance a level up. without having looked at the level before, so it is mind-warping indeed.
The lich fight, both lich and spectres. Spectres have relatively low health so some magic missiles and some vs. undead weapons work, and of course holy chant blessing (time it wisely, it does not last long).
On the lich, here a few tips:
1. He is of physical form, unlike spectres. So while ‘vs undead’ weapons are good against spectres, he responds better to things like big 2h hammers. [Note: in v1.5.50 onwards there will be more frequent vs. undead ranged weapons in shop and a quest related one. If your party is ranged-focussed this should help]
2. Holy chant skill. Your best friend. But time it wisely, lich needs to be recharging his teleport power and fighters need to be adjacent.
3. Resist life drain for your tank. By equipment or spell buff or both. Lich’s attacks use life draining damage type or magic damage so both would be ideal to have resistances against.
4. If push comes to shove: exorcism skill used by cleric (has range 2), see monk in village
5. Worst case: reduce difficulty prior to fight
6. Trickster: move past him, go up, loot items, come back later for him when you put the loot to good use.
7. You can focus on attacks that STUN as well, could make him be unresponsive and unable to teleport, attack etc. for part of the fight. Melee weapons but also slings can achieve that.
8. Lich has been nerfed down slightly in v1.5.50 (live from 3 Sep 2016), if you are on an earlier version this will make it a notch easier (teleport recharge, spectre spawn rate … all been moderated a bit)