Imagine if the Drow Necromancer was a badass mage. He had his fire mastery as good as it could get, and he super-charged all his spells (which would be facemelt, fireball, meteor shower, inferno, etc.) so they were doing so much damage it'd kill anyone one hit. The only way to fight him would be if you were a Priest spending all your mana on keeping your BUFFS going and on consistent HEALING. Or if you were a Thief and managed to sneak in unseen, and super-charge a back-stab good enough to kill him in one go.
Super strong, one-hit-kill, fire-mages would be difficult to kill. Beating their style of play would require either NEVER getting hit by them, or making yourself strong enough to withstand taking a hit from them, or beating them before they got a hit in. The only other option would be a group, a priest dedicating some of his mana to BUFFing a Warrior with good tanking capabilities, and a Hunter or Ranger or some kind of Mage to deal the damage to take out the mage.
For this to work, it would need to be necessary to have ELEMENTAL MASTERIES, which I envisage as very similar to WEAPON PROFICIENCIES. They are both types of fighting. You are good at fighting with short weapons, or you are good at magic with fire. They would have to be zero-sum equations so as you get better in a mastery to the detriment of other mastery's. Or you get better at weapons to the detriment of other types of weapons.
For this to work it would also be necessary to be able to determine the amount of EFFORT you wanted to put into a spell or a skill. It should be possible to super-charge a skill or spell by determing the percent of your STAMINA or MANA you want to dedicate to it. If you put 100% of your mana into one spell, it will do 10x the damage it would if you put 1% of your mana in it.