You thought taking out a party of adventures famed throughout the Realms was going to be
easy? 
Drizzt and company may not be unbeatable, but they're certainly deserving of
respect, and BioWare made sure that you'd have to give it to them.
On their side: I've never looked at his *.CRE file for myself, but I've heard that Drizzt is hacked in a dozen different ways; apart from his Magic Resistance, he's got something like 8 Attacks per Round, a THAC0 of -7, 200 hitpoints, etc. As far as I know, installing Tactics has done nothing to affect the difficulty of this fight. Tactics does add Improved Mages, but I don't think even Catti Brie reaps any benefit from that.
On your side: You've got all those spells and potions littering up your Inventory--ever think about
using them? Drizzt's party doesn't contain any spell-sligers, so Imoen under Protection from Magic Weapons would really befuddle them. You have time to cast spells if you organize yourself and strategize. "Enemy Sighted" contingencies would be useful here. You also seem to be emerging from the Underdark at a pretty low level--Minsc is only Level 13??? Then again, a Level 17 Viconia is pretty high.