I've noticed a few people complaining about the waves of enemies in the ratkin fortress and found it rather odd. My first time, I tried the front door and got my ass handed to me. So I figured that maybe that wasn't the 'intended' path. And sure enough, you can skip even the front door fight by going to the right. From there you can have a series of smaller fights around the back of the fort and enter from a side door on the left of the fort and skip both the big room and the crowded bar fights. By doing this you can get to the Iron Maiden without doing any fights involving reinforcements at all.
Thats the way i go to kill the boss too. Its just that my dumbass is too greedy and want their loots, so i challenge myself.
I got shit on couple Times in my old save and got my elite prisoner kidnapped. By second attempt i learn their move, reinforcement cycle, and learn to countermeasure their move, while also learn the sustainable build with good DoT damage to clear the 3rd and 4th line fast. It cost my elite cleric (she got kidnapped also).
I created a new save, because I lost all my assets to conquer the first floor, focused only to 4 adventurers, manage their quirks, grinding their level fast, train only my warrior to all 20 stats, pick only skills that i needed, ignoring chest mimic in the dungeon to prevent cursed gear on me (i never change gear except for weapon).
Combat note:
I maxed out the stacks of my block and block+ on my warriors, guarding others so that all enemies would always aggro to my warrior. At max tokens, i start poking the the 4th line with iron swan or 3rd line with DoT attack (i forgot the name, it was skill with brown color).
I used my paladin to spam Faith blessing till it regenerate 9hp perturn, so That's mean i spam 3 round to get all my adventurers regen to 9 hp perturn. Other than that i used my paladin to Shield my warrior when she get overwhelmed. The good thing about paladin is she got more HP than most warriors, my veteran paladin got 66 HP alone while my veteran warrior only got only 45 HP. So my frontline is pretty much impenetrable while they can't touch my 3rd or 4th line because of my warrior and paladin guard.
I used cleric to spam razorwind, the skill got decent damage and pretty good DoT, do some healing, and cleanse annoying debuffs if necessary.
I spam mage skill that attack 2-3 enemy line (i forgot the name) combine with razorwind, the DoT alone inflicts more than 15 HP per turn in the 3rd line