Jewel of the North
I'd do the opposite and have the drows move closer to the entrance of Undermountain. I dont know if there is a nice ambush spot in the first few levels? Or have them somewhere in Skullport, going for a more Urban encounter.Perhaps T’rissa pulls back to aid her sister on level 4. The drow on that level are cut off from the bulk of their forces on level 10 and are slowly being picked off by an aboleth calling itself the Lover in the Dark.