Yup there are two ways to do this. The first is using the skill damage cap (max possible damage it can do).
skill_damage_cap: {
//Exemple Below caps firebolt damage in maps within this zone to a maximum 50 damage,
// (depends on HMAP_ZONE_DAMAGE_CAP_TYPE in src/config/core.h)
// when cast vs players and monsters.
//MG_COLDBOLT: (50,"PLAYER | MONSTER")
}
The second is to use the mapflag adjust_skill_damage.