Corners and Fitzroy are only fought in the All Bosses category because the rewards they give don't count towards the in-game % tracker (which you can see at 1:38:42)
@@zekoop3740 9 months late, but for Fitzroy you need to find all of Letica’s locations, then head to that water area inbetween Blockrock and the Desert. At the bottom of the largest room there is an entrance to an underwater room, which you will absolutely need the tweaks that increase oxygen for. At the left of that room, if you found all the Letica locations, there will be a schematic, a bed to recover health and a small entrance to the left that will eventually lead to Fitzroy
