Hard Fire Knight stage 10 is most likely the hardest content in the game right now due to the amount of different roles required to manage damage and shield mechanics.
There's currently 2 general approaches to getting the shield down and dealing enough damage with the shield down to kill Fyro and survive the trip.
This approach combines the elements of ally attack and counter attack, but also requires block debuffs (cleanser is not sufficient). Most notably, it requires Cardiel with no substitutions and is therefore not accessible to many players.
Counter Attack with Ally Attack
Double ally-attack compositions are more straightforward and have a slightly wider viable champion pool. It currently requires Gnut (July 23' Fusion), 2 ally attack champions, and a cooldown reset with turn-meter boost. Similarly to CA+AA, you're currently locked in on using Kymar in order to also fit sufficient enemy max hp based damage before the boss reclaims his shield.