Got the bow and arrow in level 2, why can't I find that last key? Also, how can I access the SE bottom floors?
There are twelve keys in total in level two.
To get the boss key you need to access the stairs that take you to the bottom floors.
To do so, bomb the upper wall of this room.
You will get into a Triforce room. Then bomb the wall on your right. Go trough two more rooms and you will reach the stairs.
Hope this helps.