Just played again yesterday, and found out how to reach the hidden boss. Needless to say, I wasn't even remotely close to having enough stats to beat it even after collecting all items/gold/experience. Are you sure the boss is beatable on a normal run? I don't think it's possible
ps. Also, are there secret passages in this game? I couldn't find any, and it also seems I couldn't find any kind of secret item either, e.g. to counter magic damage from mages
I checked all the items, and the added stats are all correct. However, I found that when you give the cross to the fairy, she increases your stats, and this time it is a percentage increase. Since there are no prompts for this in the game, you might have misunderstood. (Spoiler: it increases by 33%)
I submitted a new version(v1.1.1) with the corresponding prompts and also fixed some missing localizations, hope it can help you.