The Staff of Arcane Blessing you get from the wizard's tower in the Underdark has a special version of Bless, Mystra's Blessing. Having guys up on he bridge usually helps me in this fight, its better to get there before they do at any rate. You can get another Wyvern poison...from "Stickshit" the put upon gnome? I think. I also wouldn't underestimate some good old grease and fog cloud, anything to buy your guys time while you take care of the spell casters.
I've never figured out if you can attack them before Lae'zel leaves, because I consider the conversation they have important to have, but you could always take her out of the party, or deceive them during the conversation.
Grease is actually a good one, easily forgotten/overlooked. Fog, just as well, but the one problem I have with fog is that your own character becomes useless in it as well.
More or less hoping you`ll later on get some goggles/glasses/mask that provides immunity from blind, if only specifically for Fog, to make it somewhat more appealing. I'm aware it's mainly used defensively, but it does have this pop smoke + use infrared goggles vibe to it.
As for taking Lae'zel out of the party is not an option for me, as she's the reason why we're going there in the first place.
