Here's how I did that level after many tries at holding the fort in the 'library'. In short, compartmentalization. Close up the bad guys in small areas that they can't get out of. So, close all the doors on the level before the monsters go hostile and before the high summoner can spawn off a horde. MOST monsters/bad guys don't open doors. The only NPC's I've seen opening doors are when you trigger an event and they unlock/open a door for you, or certain NPC's in towns that open doors to houses then go inside.
I talked the head guy at the door into allowing me to walk around the complex. Take a thief and use stealth ALOT (some doors appear to be locked in the open position so you might have to pick locks to close them) and close ALL the doors you come across. Close doors behind you if you go through a doorway. This allows you to lock up as many bad guys as possible so you don't have a constant stream of summoned monsters coming at you. The first place you should cover should be the door to the high summoner on the east side of the map, if that door is already closed then come back to it later. Once you've closed all the doors you can, park the thief in hiding near the door to the high summoners room, then have someone from your main party talk to the priest at the entrance and set off the attacks. I'm not sure, but I think the door to the high summoners room opens automatically once you trigger the attack, if it remains closed, then you're in luck. If its open, then run over and close it fast. You can run the thief back to the main party to join in the combat, but make sure you close all the doors behind you. The battle in the main hall will be much easier. You will only have to deal with Eldathyn, the priest and the 'librarian'. There are still a large number of Eldathyn, so a couple of webs on the doorway will make life easier. I've developed a stratedgy of firing 2 web spells into the same area, one from my mage and one from the bard. It doubles your likeliehood of trapping the enemy.
Then, for loads of fun, release the trapped adventurers to roam the map. Open a door to let them into a room, then remain stealthed so you can 'activate' the battle. Oddly enough NPC's (blue or summoned) will NOT fight if you can't see them with one of your characters. You have to be able to see them for them to initiate attacks (they may continue until their target is dead even if you can't see them). So basically follow the group around, when they run out of creatures to kill, follow them out of the room and close the door, then follow them around until they come to another door and open it for them and let them go in first. This is highly effective for the high summoners room.
The high summoner is the most annoying so you may just want to allow the adventurer party into that room then watch them tear up the high summoner. Once the high summoner is dead you can close the door locking the adventurers in the room. That allows you to take out the rest of the monsters on the level alone (you don't get experience if they kill the monsters for you).
I personally let them do all the work on this level as there are some concentrations of Yuan-ti and Yuan-ti priests that are annoying. I think I did take on the healer in the middle of the map alone. I allowed the adventurers to do their thing because I was bored. I just had to keep my stealthed thief running around the map to keep activating combat. Once the level cleared, cast a few web spells at these guys (who are conveniently clustered up, then use a few area of affect spells and follow up with missle weapons. I know its 'evil', but they have some excellent equipment and you suffer no penalties for killing them.