how the hell i supose to defeat all endless waves of mobs in noth castle???
cheat. it's a skill, legit option, you find it in the skill tab. that's why you first get asked "do you want the (optional) cheat skill?" when you start a new game. the battle system is not balanced, that's low priority, that is why the dev had this cheat skill built in. it's officially legit and lets you win without wasting resources.
if you rather want it to be a challenge, and be much more work, you can just choose to not use that skill, but that's going to be tedious painfully long fights for each and every smaller skirmish.
there is even the option to toggle if the skill should be available later in the game, in case you started without it.. iirc the old woman had that setting? don't pin me on that i always used the cheat. i mean, it's legit. right? dev says so.