Lets see the ability description :
The first player may make a hero attacking this Cave Goblin reroll any number of their attack dice. The new result must be accepted.
I'm not sure about the interpretation of "reroll any number of their attack dice" :
- Does "number" mean for example : "Reroll all your 4" ?
- Does it mean "Reroll that, that and that dice" and the First player choose ?
- Does it mean "Reroll 3 dice", and the attacking hero choose the dice he want to reroll ?