Actually I finished this.
Even if you finish the dragon quest, you still have the purging wand (although now useless.) If you've sold it, thankfully there are only a few vendors that are worth trading with so tracking it down shouldn't be a problem (Only time consuming).
From there you make a dialogue with the seeker dude (Can't remember his name) and that will trigger them being at the fort site, waiting for you.
Once you get rid of the shriekers (via dragon spell) you can then insist that the seekers give you two men to fight with you. (I personally went for melee fighters as this next tip will explain)
Then I went to the right hand side of the building. There's a series of arches here and you are somewhat sheltered from most of the attacks. Before you even think of starting the battle, make sure you have plenty of resurrection scrolls! And hopefully you have the feat "Morning person" to rez to full health!
Instead of triggering the dialogue with the bad guy, I threw a couple of bombs up and over the wall to trigger the battle. Alexander will try to blind you, but if you are far enough behind the wall, it doesn't hit you. The bad guy fighter will phoenix jump over to you and the rogue will climb the ladder to you. The main problem is that unless you all happen to be in this little nook, arrows/spells won't work if your character is on the right side of the archway. (Hence why I suggest going for the melee support rather than archer or mage) These guys will try to buff you first rather than fight, but when they do, they'll run out into the main area, and not get taken down by the bad guys (Who, for some reason ignore them and attack only your group) Hopefully one will make it to the enemy archer.
Now, here's the tricky part. Knockdown arrows/ground slam/ battering ram DO NOT WORK so save your turns by using ice spells/arrows/bombs as the bad guys will usually trip trying to get to you.
When you can, move all your troops out of the alcove and onto the area just to the right so that the mage and archer can't get to you. Hopefully you can now off the bad guys one by one until the drill worm comes.
When the drill worm comes, it'll take down the enemy archer first, giving you time to chip away at Alexander's health. When he pops up in the middle of your group, he'll knock down Alexander so take him down first. The drill worm will then keep trying to take down your guys but if you keep rezing them, you should be able to chip him down gradually.
Long winded, I know, but it works!
Happy hunting!
