My solution for Castle Storm was the same I use for every other place: I always travel with 100 mid hp potions, and 100 mid mana potions.
Against the Executioner, after shitting my pants running from the beheaded ones and fireballing the ones that pursuited me, and getting that sweet surprise of a tough boss at the other side, I got super lucky that he got trapped in a tiny corridor at the corner, while I was freakingly trying to find the secret door to the second level. At first I thought I was doomed, because I walked inside that "invisible" corridor (which Im still not sure wasnt a bug or not), pushing Griss (I wanted to stay away from the beheaded, just in case), so I actually got trapped with the damn thing hurting me while Griss was in my back. But once I was trying to run on panick mode, I hit the end of the way, and like always happens, switched places with Griss, so I got the perfect cover with me in the wall, Griss in front and the Executioner in front of her, unable to turn and get me. So I paused the game, took a deep breath and managed to kill him without further stress. Beginners luck, praise be the Three. And after killing the damn thing and not finding the door, not even with a scroll, I exited and felt pretty stupid to find out that I was looking for the door at the wrong place, trying to reach it in the far corner of the map, while the door, while in that same wall, was located far earlier than at the far corner, where I trapped myself with the Executioner. I mean, I almost killed myself in the dumbest way.
Still, after my potions stocks fell from 100 to a little above 30, I started to get worried. But at that point I had already cleared it all out, and only the 2 baddies were left, and compared to the rest those were a breeze of fresh air.
But undoubtly that was the toughest dungeon I've been in the entire game. Granted, some other may be harder, but any other dungeon that proves too tough, I just give up temporarily (or at least stockpile tons of potions and scrolls before trying again)