Never thought of using marshals + bards. Anyway, definitely focus on on killing the xenomorphs w/ranged attacks, as well as slowing them down w/spells, traps, tanglefoot bags, and undead mooks. And run from the slaymaster when it rears its head. Hopefully it'll be too distracted with massive amounts of shambling corpses to run you down and turn you all into piles of gore.
Edit: I will say, it's probably not as bad a situation as storming the tower of a reduced kobold lich .