This is not supported by RAW. RAW causes the ash effect the instant 0 hit point is reached.
It astonishes me how many people quote RAW who have obviously not read the rules -- the description does not say that the target is 'instantly' disintegrated, only that it is disintegrated "f the damage reduces the target to 0 hit points", as I quoted in a previous post.
So a more accurate reading of the RAW would be:
- deal damage
- once damage is dealt, see if target is at 0 hit points as a result of this damage
- if yes, target is disintegrated
It's also telling that you are explicitly not following the RAW of the Wild Shape abliity, which tells you explicitly what to do when the beast form's hit points go to zero.
But, to quote the Angry GM, you can run your own game any wrong way you want.