Inis
clerics and druids can still cast spells - it's only mages who are prohibited.
You could try prepping your party before they leave the inn whith the standard buffing spells - haste, prot from fear, chaotic commands, bless, stoneskin and such.
When you exit, send one fighter towards each enemy spell slinger, and another towards the archer, to draw his fire. If Jaheira is in your party she can cast insect plague - a cleric could try Holy Smite.
If things go poorly, your hasted party might be able to just run and get to the edge of the map. Go to the govt district and see Corneil in the council of six building about a license.
Or you could try to kill at least one of the Harpers, then reenter the inn. If they don't follow you, rest for 8 hours, then go out and try again - pass the gorgonzola, please
Kez