I can confirm the dragon fight is impossible to replay, however the trophy is still not missable because the area is still accessible and you can hit yourself with the crystal outside of the dragon fight and the trophy will still trigger. (Confirmed, just happened to me)
For anyone having the issue where the crystals are not falling after you beat the dragon or when you come back after you have finished the level, connect your PS4 to the internet and the game will have an update - this update fixes this. I had the same issue where not crystal was falling - updated the game and it was fixed.
Already beat the dragon, went back to get this trophy and I'm on my 3rd restart chapter and for some reason the crystals are not falling when I hit them!?