Lichs are powerful undead magic-users that can be very dangerous to an underleveled or unprepared party.
Battle and Strategies
They open battle with a casting of Protection from Magical Weapons and Spell Trap. This combination can be very dangerous because lichs are immune to normal weapons. Then they cast Time Stop, release a spell trigger that includes Death Spell, then cast Symbol, Stun and Symbol, Fear. They also like Sunfire
It is recommended that Anti-Undead weapons be used (Mace of Disruption, Daystar, Harrower and the Foebane) along with a few Arrow of Dispelling for the magical defenses that these individuals will have.
Locations
At least two are present in the ruined temple of Amaunator. They are accompanied by two Skeleton Warriors and two Greater Mummies. One is in the room right after the room with the central lava pit. There is another lich in Sendai's Enclave, in a room that includes the Eye of Tyr, the Skull of the Lich, and the Heart of the Damned. There is another lich in The Cult of the Unseeing Eye in the Athkatla Sewers. This one is linked to the Restore Kangaxx's Body quest. Also, the other lich linked to the Kangaxx quest is in Delosar's Inn in the Athkatla Bridge behind a trapped door. Another lich is located at the Crooked Crane Inn at the Athkatla City Gates.
Combating and defeating
Liches are fond of using Disintegration, Imprisonment, Power Words and Symbols. Counter these spells by using Death Ward and Spell Immunity: Alteration (stops Disintegration), Conjuration (stops Power Words and Symbols) and Abjuration (stops Imprisonment, as well as Breach and Remove Magic, which they tend to use to make your party more vulnerable)
In Baldur's Gate 2, an Inquisitor's Dispel Magic was often enough to undo most of the protections afforded by the initial spells casted by a Lich. Alternatively, Breach and Pierce Magic can be used to rip down spell defenses, though without removing your own buffs.
The Carsomyr provides similar powers to the Inquisitor but can be equipped by any of the Paladin classes. Additionally being a + 5 weapon, it's unaffected by spells such as Absolute Immunity and Improved Mantle.
The Level 7 Clerical spell Sunray is effective against liches, dealing (LVL)d6 damage to undead and forces them to save or be destroyed.