I defeated the Dark FF using only Cap! On hard level. Here's how.

[Lightwave]

I used the WW2 outfit to get +24% health. I used Heroism to lessen the damage I take. I equipped Attuma's Wrath to dish out 25% more power damage. At the very beginning I used my extreme attack then immediately switched to Spider's Bite for get away speed. I run up the stairs then used my Heroism boost. I used Shield Throw a few times as they came at me then jumped off the ledge and with all my speed, rushed to the diagonal stairs and to the ledge. This brought the Dark FF to the base of the ledge under me. I then went to the railing by the stairway and as they tried to come up the stairs I nailed whoever came up with Shield Throw and went back to the middle of the platform. This caused them to track me by going back down the stairs and under me. When I found this out I switched to Attuma's Wrath for more power damage and repeated the system. Also I was able to wear them down when they would stray into the open where I could nail them using Shield Throw. I took quite a long time but when I got down to Mr. F, I just jumped down and beat the crap out of him. His health was low by that time anyway. And that's how I did it. TA-DAAAaaaaaaaa!