I have run this adveture twice now, and one group had only one fighter in it, so they were forced to devise group tactics, and relay on summon spells. The thing was that group did better than the warriors.
I find it all great fun though, and have throughly enjoyed running it both times. We had two PC's die from stupid actions, but mostly the group was capable, because they always posted something to help themselves, simple first level spells, Alarm at the entrance so they had time to prepare for invasion, etc.
You must remember, your not the only one with tactical abilities, your Players are as well.