It isn't a hidden boss its just the tier 3 boss, there a 3 different routes and bosses in this dungeon.

Fast way to tier 3 boss:
You dont have to open the bottom gate to get to this boss. Just get 4 photocells and go to the top gate and hit it. go past and drop down to next area and collect 4 more photcells. RECALL back to beginning to run back to top gate and hit it gain. Opens path to tier 3 boss.

Did this a few times last night. On one run we got 4 drops at the end =D.