Hard mode FK needs some tuning
So, having finally reached FK10, I feel I can provide some valid feedback. The boss is tough, as intended, but there's a few things which make this fight just feel unfair.
For starters, the elephant in the room is clearly the janky Raid AI. They obviously hotfixed some champs, such as Coldheart, who will specifically use their A1 instead of their A2 and A3 while the boss has the shield up - but everyone else is left to fend for themselves. This has obvious consequences when relying on certain debuffs to make the fight more manageable - such as, speed down. That said, even with this issue fixed, I don't think it's enough.
As it stands, you basically need to solve three problems in one. You obviously need to bring the shield down in order to do anything. Further, you need block healing, or else you won't outpace the boss regen. And, lastly, you need some way to actually stay alive.
For anyone without either Valk or Cardiel, those problems are practically unsolvable. But let's make the assumption that you do have one of those. My current team is Valk + CH + Helicath + Fatman + Kimi. Everyone except Helicath is at 305+ SPD, and I have an extra +6 speed from GH. I actually am able to bring the shield down, and, on manual, I think I'd be able to win pretty consistently. On auto, though, it's practically impossible, mainly because of the aforementioned AI problems, but even with those fixed, it'd be dicey.
The reason for that is mainly because even with all those other problems to overcome, there's two other ones that just make it even more difficult - namely, you need enough accuracy to land your debuffs, and you need enough resist to stop the boss from placing DEF down + SPD down.
My suggestion is quite simple - just make the boss a bit slower. That should solve all of the above problems. I don't know what the boss speed is right now, but I'd suggest dropping it by around 25 for starters, and seeing how things look after that - but possibly as much as dropping it by 50.