After that bit annoying encounter with that ranker, I began looking though the floor and trying to find the boss room number 4, since the key number 4 is the only key I'm currently missing.
It did took sometime and the next boss room I did encountered was wrong one since I had already the key from it from that ranker guy… And the next room after that had also gave me an double…. Shit.
But at least the third fuking boss room gave me the key I was looking for. And yes, I did ended up encountering the guards defending the gates and yes, I brutally murdered them to the point they screamed for their mommies. And by the way, I didn't get attacked directly again…. Maybe they got alarmed or/and scared, causing them to be wary of me.
After getting the final key, I didn't went straight to the floor guardian room and instead decided to log off and take a short rest.