Basically, spell up outside the building then go in. Hit the lich hard and fast before he can cast a lot of his party-messing spells. After the lich is gone, Go after the Beholder and then the Mage. The thief will continually try to backstab your mage (at least to me
).
I would attempt this quest at a party lvl of around 13-14..I did mine at around 12-13
Happy Gaming!