I feel both bad, and a noob now :(
Noob, because I could barely kill him with act 2 chars, and bad for being marked as a solution ><'
I'll find the solution, I swear, and post it here >_<

Okay, I've opened the game, and searched the place:
For the bottom-left room, you will need a dark-magic spell, called "Whispering Shadows". It allows you to see secret doors. There is a secret door in the bottom-left corner, that needs a character with high might to open.
I think, the enemies inside it will give you the key for the NE door, if not tell me, and I find the key ><
(And the doors I mentioned earlier, the ones I was having trouble with, are on the 3rd floor...)
Sorry for being confusing, it often happenes to me that I try to help, and I just end up messing it up even more :(

I'd name it RPG of the year if no better RPGs come out this year.

You must activate the game on uplay to activate that dialogue line.