I just did the Lich fight the other day, actually. I was still in Chapter 3 with a ~level 18 4-PC party, with two tanks and spellcasters being one 3/4 Priest/Mage and one 4/4 Priest/Mage whose only level 4 mage spells were Capture Soul and Simulacrum. Didn't have any fancy items other than a Goo Bomb, a couple Strong Energy Potions from Egli and some spare Weak Strength and Speed potions for the tanks with an Ambrosia for the spellcasters from the Gremlin Wine in Chapter 2.
First round would consist of tanks using the Goo Bomb to slow down most of the enemy and block vision of Lich and popping strength potions, 3/4 caster to Slow Group, and 4/4 caster to Haste the 3/4 caster. Second round, if the Lich didn't Firestorm the party, there'd be a Haste to the 4/4 caster, and two Simulacrums of Vahnatai Keepers. Third round is the free round from the Slow, so I'd try and Capture Soul at about this point. (It's more than possible: I'd usually snag the Lich in about three tries, with ~Lv.18 ~11-13 Int. spellcasters. God bless potions for multiple tries.) From there, I spammed Lich and Vahnatai Keeper Simulacrums inbetween Slow Groups and Curse Alls and made short work. There was a fair dose of luck in the behavior of the Lich, though: if he managed to get a pair of Firestorms off, or a pair of Kill spells, then that just fell apart. The Slow and webbing did help a bit though, enough to slow down the spellcasting just enough so that I could recover whoever was in direst need.
I don't think you'll have access to a Goo Bomb, though, so I'm not sure how to best approach it. Probably you'll work with 6PCs, so you'll have extra spellcasting on your side for more Simulacrum spams. I find Vahnatai Keepers to be a steal at 8MP and a constant source of Demons and Kill spells. And Lich summons, although a hefty 20MP apiece, are well worth it, especially when mitigated by Energy Potions.
Where were you finding Fire Orb Necklaces, Shields of Khar, Shielding Crystals, and Lifesaver Amulets in Chapter 3? I'm quite interested to know - I sure didn't see any in Mancuso. : )