By the way, as far as I remember from my first playthrough of this mission, towards the end, not 45 paladins join, but only 4 – if you play on Recruit difficulty.

45 is if you open the dungeon for the dwarf artifacts, and 4 is on the map itself :) That's how it was for me, I just finished this mission. The map is actually quite easy. My Frieda is already a strong fighter, with 17 attack and 21 defense. I also upgraded the "Vigor" perk, which increases damage by 5% multiplied by morale. My morale is currently +3, which gives a +15% damage bonus. Combined with the 15% attack bonus from the "Expert" skill, it results in 30% more damage in melee combat.