Has anyone been able the beat the Dark FF with only Cap on hard level?
No that is way to hard otherwise being Wolverine or other character that has health regeneration such as :x-23: :deadpool: :stooth: or characters that can heal by using there powers such as :strange: :emmafrost: :spiderwoman: and powerful characters such as :storm: :hulk_icon: :colossus: and maybe others. :storm:
There is a way to make it to a ledge and have the entire Dark FF under you. But if you don't have a targeted attack or a radial energy emitting attack, you're toast.