Use algorithms instead of hard numbers.
a very rough example:
Health = CON * Level
Mana = INT * Level
DEFENSE = Armour_Value + CON + RESISTANCES
Spell_Damage = Spell_Power * Level + INT
Actual Damage = DEFENSE - Spell_Damage
This way, no matter what level your mob or player is, the damage will scale and work with any items/resistances equipped.
However, you WILL need to plan these algorithms to work in the long term.