So it took us 4 carts of oil soaked wood but we killed it. Opened the door shoved cart 1 in and closed it, the quasit had her sinspawn push it into one of the rooms closet, the sinspawn were already weakened from previous encounters with us so the fire finished them off. The second cart was quenched by a water elemental the quasit summoned. The third cart was pushed by the quasit herself into the second closet and closed off. The 4th cart she had no way of dealing with so it filled the room with smoke and after a lot of rounds she eventually died from smoke inhalation. The carts, wood, and lamp oil used ended up costing less then half the 1 scroll of detect invisibility I purchased. :)