bon j'ai cherché un peu sur le forum et visiblement c'est pas la première fois qu'on pose la question 
j'ai trouvé ma réponse détaillé
KoalafiedKiller
On Just the Story and Sword and Story you get 100% experience. On Blood and Broken Bones you get 90%. On Death March you get 80%.
On Just the Story enemies have -50% health and do normal damage.
On Sword and Story enemies have 10% more health and do 40% more damage
On Blood and Broken Bones enemies have 20% more health and do 130% more damage.
On Death March enemies have 80% more health and do 230% more damage.
And just for reference as to what "normal" health and damage are. Group type monsters gain 80-90 essence (health) per level and 5 attack power per level. Solo monsters gain 240-260 essence and 40-42 attack power per level. Humans gain 55 vitality and 20 attack power per level.
The Last Wish is a very low level quest for where it is in the story. I believe it's level 14 which is strange since the quest to even get to Skellige is level 16, and The Last Wish is one of the last quests you do on Skellige in that chapter so you outleveled it by a good amount.
https://steamcommunity.com/app/292030/discussions/0/350533172689695342/