it’s not… you always need to get to bottom middle room in order to access boss room
graphic in the background might be misleading 
given dungeon battles are like in picture below:
| NW | N | NE |
| W | M | E |
| SW | S | SE |
Possible movement paths are:
W-NW
W-SW
SW-S
NW-N
NW-NE
NE-E
E-SE
S-SE
S-M
Shortest path is:W-SW-S-M
Other way is generaly:
W-NW-NE-E-SE-S-M (skipping treasue room in N, E also is treasure room, but i’m not sure if it can be skipped or not)