I'm doing a pro-rebel playthrough and so far, I got to the following position: https://imgur.com/a/Dso2r (Char / Map / Quests). All my creatures are level 23-25.
I received a mission from the servile in charge of the Safehouse, where he asked me to kill Moseh. He also warned me that East Barrier Zone is my safest bet because the other Barrier Zones are heavily guarded (and they indeed are, or at least the Thornton ruins).
As per his suggestion, I cleared the area, disarmed the lightning traps and the mines, and got into Moseh's bunker. Going downstairs, I found myself on the northeast portion of the map, where I killed a Battle Alpha. When I got near Moseh's chamber, the game gave me a warning that I was about to face the Shaper. Moseh imprisoned me, and started draining my forces, but Khur sacrificed himself for the rebel cause, and in the end I only lost 1 point of strength before the fight really started.
Here is where I can't get through: No matter what combination of spells that I cast, I can't seem to kill him. I tried to open the nearby doors, but they are stuck.
After dying multiple times, I managed to get into a comfortable position with speed spores, war blessing, and other buffs/attacks. I also leveled Battle and Blessing Magic to get access to Lightning Aura/Spike Shields. That is when I managed to drop his health enough where his double heals couldn't bypass my damage. To my surprise, when I got to about 33% of his hit points, he triple healed, and that is when I knew there is no way I can beat him using this strategy.
I searched the subject around, and everything points to me "repairing him" and "helping the Shapers", but sincerely, that is not what I want or had in mind. I have seen a video where someone access the bunker through the southwest portion of the map, but I have no idea how to get there.
How do I defeat him? Is it possible to disarm whatever is empowering him?
EDIT: I'm playing on Normal difficulty