Although quite hard it was definitely not impossible and by no means unfair. Actually, if you do it right it's really easy.
Our first try failed because we obviously didn't know what we were getting into, we were lvl 8 and decently geared. Mage killed us a few rounds in with meteor shower.
I started the fight out with my enchanter lizard i placed her and my archer both on the left tower wall (2nd floor), as soon as my turn started i teleported Alexandar far out down the road to the boat and nuked him with an oil field to prevent him from going anywhere and from blinding the entire party (and doing crazy damage with it), then used my archer to CC the Mage and the Archer on the opposite wall. My brother, phoenix dived in with his melee character, blood sacrifice and raged, then 1 shot the archer while he was immobile and made sure he stayed out of range of the melee guy who's capable of a one shot if you got less than 300-400 hp. He moved the red prince in position to gang up on the melee enemy. The enemies pretty much did exactly as expected, the Gheist tried coming up to my Enchanter and so did the Mage and the Melee guy all standing close to each other. In my turn i nuked them with frost shards freezing 1 of them and chilling the other 2. I cast infection, everyone around me gets poisened, i continue with bless to buff myself for the next round, my Archer buffs adrenaline to get 2 more AP, shoots a poison cloud arrow and a fire arrow down to the 2 chilled guys making them explode and burn and then using electric discharge to stun both of them for the next 2 rounds. My brother buffs himself a bit and skips his turns to get more AP. Back to my Enchanter, staff of magus, fire breath and another electrical discharge to create a electrical steam cloud. When the enemy mage was finally not stunned anymore he immediately started bombarding me with fire balls, unlucky for him he caused the small gas cloud close to me to explode and burn up the entire second floor, which was immediately (for some reason, not sure if it was the enemy or something i did) transformed into holy fire, essentially making me and my archer invincible forever. Gheist and Melee started moving but immediately got stunned again. Brother phoenix dived down to the red prince getting into position throwing a oil field for the soon arriving Alexandar. I continued blowing the Melee, Mage and Gheist up with my ready spells and some grenades/arrows combo-ing as many fire, poison and DoT's and CC's as possible instantly killing the mage. As Alexandar finally arrived in the castle again he was immediately teleported into the oil field, followed by a inferno bottle, poison cloud arrows, more oil bomb skills and a stun arrow making him essentially useless for the next 2 rounds. Gheist and Melee moved, got burned and died both before reaching the second floor. Alexandar then got raped by our melee characters and knocked down again when he got up (300-400 damage per melee swing). After Alexandar died the sandworm popped up which was basically just a combo-fest, all damage he did (his damage is really shit) was instantly healed by holy fire beneath me.
The key in this fight is to trivialize Alexandar and get the mage killed or at least stunned as soon as possible and as long as possible to prevent him from casting the meteor shower which is essentially a party wipe.
All in all i found the fight one of the best, if not the best (the other one being the dweller one you complained about which we easily finished being underleved 2 levels.)
