tips on no casualty run on impossible difficulty

[bucazaurus]

If the hit was critical damage the game will ignore the extra damage and only return the base damage instead. Also the extra damage made to glot armor will be ignored.
In your case since the dragons have fire damag and thorns are vulnerable to fire , the game ignored thorn's penalty to fire and returned the base damage.
10k halved is 5k and since the pain mirror returns more then 100% you end up with 6k