So get this
I played the game on Difficult mode and went straight to Air Man. I let him take a few pot shots at me (about two or three) before I started fighting back. I still kicked his ass. *shrug*
Nearly every boss in Wily's Fortress has some potential to kick your ass. Mecha Dragon's flames aren't particularly damaging, but sometimes you can glitch and end up in knockback far longer than you should, which pushes you right off the platform.
Picopico machine isn't something most people regard as difficult, but it is still pretty easy to die from when you don't know where the parts are going to come flying in from.
I think Guts Dozer and Wily Machine are probably the easiest of them all.