Atheon, there is very little room for error in that fight. Lose one or two guys, you'll be riding the struggle bus. Skolas would probably the second hardest because his taint ability and the ads. Also, if you lose one guy in that fight, you're going to have a bad time. It's not as bad as Atheon, but it's by no means a breeze. Crota is the easiest one. Sure, when someone dies, you have to destroy the Oversoul and it's pretty strong on hard mode, but it can be taken down. As long your group has rockets, and your swordbearer doesn't die from Boomers or Crota, it's breeze. On hard mode, you can easily two sword Crota. This is my opinion, which I have formulated through my experience with this game. Yours may be different from mine.