Hello everyone!
I'm currently playing through the Paragon map, and it's amazing. Huge respect to the author, and may they have lots of good karma. I couldn't resist registering after seeing this post.
I stopped Mogrein before Deis, one tile before the castle. I used Disguise, and he attacked on his own :smile26:. In an open field, he's easily defeated – just equip the Cursed Armor to slow him down and take the Sphere of Inhibition (red orb) to prevent him from casting spells or trying to Haste. That's it... Then, just run away from him with a flying unit across the field, and shoot him down with ranged attacks.
Another tip: it's worth equipping the Spirit of Inhibition so that his morale doesn't suddenly drop.
Added 3 minutes later
I didn't even think that after my description of how to play through the Paragon map, there would be such a lively discussion and so many people trying it out