The end of HotU Chapter 2 battle is hard.

I'm playing as a solo rogue 12/assassin X/shadow dancer 1. Very strong build, pretty cheesy. I just wanna see the plot stuff for now, having never played. I'm soloing. (I've solo'd SCS BG–BG2: ToB.)
I understand this is a bit niche, but is it actually supposed to be this tough?
I just wanted to relate how I was surprised by the difficulty here. Every other encounter up to this point was a cakewalk.
Also,
But when defending The Seer, I can't do a thing. I can one-shot enemies one every round no problem, but there are just too many to kill before they kill The Seer in two rounds. And I defeated all the possible allies! This was the smallest war party!
I understand this is a bit niche, but is it actually supposed to be this tough?
The "squads" I defended with don't actually do much. Also, they don't attack unless they're attacked. They're overwhelmed almost immediately. Also, it seems like any progress you make in defending those coming through the gate doesn't matter at all compared to the huge (and tougher) group that come through the docks. Eventually I had to make The Seer invincible to get past it all. I know it's probably the AoE element.
I just wanted to relate how I was surprised by the difficulty here. Every other encounter up to this point was a cakewalk.
Also,
Are the rebels usually such pushovers, or is that some EE bug?
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Comments
From what I remember the reason why the second part (defending the Seer) can be tougher than usual is if you fail to defend the first part (defending the fort). If you manage to repel the attack in the first part defending the Seer becomes easier.
From what I remember for the first part you should put your archers on the high ground of the fort and have golems guarding the courtyard. You can probably get the angel and the gladiator to fight in the first part but it probably won't be necessary. If you find that you can defend the first part without relying on the gladiator and the angel you can have both of them defend the Seer when the second part happens.
I hope this helps.
Yes, the AI on your side is pretty poor, and you're right that you'll have to do most of the work yourself. But I didn't even realise the Seer COULD die, so something's definitely going very wrong for you.
My guess is that you've failed to "prepare" adequately...
* SPOILER WARNING * (obv :P)
Each of the factions that you remove from the picture prior to that battle starting will improve your chances. Since all BW builds haven't even heard of Will Saves, the Mind Flayers are probably the most desirable set of enemies to remove. The Beholders would probably be my next choice.
For your char, I'd take the Undead or the Golems as the "last" (see below) group to knock out, for obvious reasons. (Probably the Golems, because they're useful allies whereas the Deva is dead weight).
Again: these are things that you need to do BEFORE the attack on Lith Whatever starts. If you've chosen "badly" or messed up somehow, it's too late for that and you'll just have to do what you can.
IIRC, the biggest problems with the fort defense are the "summons in the courtyard". Your troops seem to either not notice them at all or get oneshotted. So keep yourself in there, and backstab those squishy things into little pieces. With that char you should easily be able to kill each one in a single round, which will keep them from causing any damage at all.
Good Luck!
* DEFINITELY SPOILERY WARNING *
If you're smart, you can actually remove more than 3 factions. Think about the geography.
I admit I am also confused as I've never had any of these issues...but I'll investigate.
That said, I've beaten HotU solo with both a pure Fighter and a Weapon Master at a minimum, so I'm not sure that was the problem...