I don't think that's a good idea. Mostly because I'm betting the Merlin wouldn't have any trouble beating down someone with Mythic Toughness.
And the 'problem' is more or less the same (if to a slightly lesser degree) for non-magic users. Or at lower levels of play (examples below). It's a feaure, not a bug. Attacks are lethal in this game very intentionally, particularly those of supernatural creatures. You get out of the way, or you get seriously hurt.
A few examples:
High End Physical: A guy with Mythic Strength and a car as a melee weapon (they count as Weapon: 5), hit's a guy with Mythic Toughness. He's doing a minimum of an 8 stress hit after the second guy's natural armor. Now the Mythic Toughness guy can take a few of those, but not many, and if he hits by, say, 2 like your example? That's a 10 stress hit and the Toughness guy can't take more than one of those before Consequences start piling up.
Low End Magical: Harry Dresden (ala Storm Front) goes up against Victor Sells, Sells fires his Killing Flame Rote at Harry and Harry gets a hit. If he's hit with 0 shifts, Harry'll take a 5 stress hit, and enough that he'll need to take a Consequence not to be taken out. If he's hit by 2 (like your original example) he'll take a 7 shift hit (only two lower than your example) and need a Moderate Consequence to avoid being Taken Out. The same applies to Harry's attacks on Victor.
Low End Physical: Meryl has a large axe (Weapon: 3) and is fighting a Red Court Vampire (Weapon: 2 Claws). Both have Inhuman Strength, though the vampire's a bit tougher. If Meryl hit's by 0 her attack deals a 4 stress hit and the Red Court can just barely take it without Consequences, if she hits by 2 it's a 6 stress hit and the vampire needs to take a Consequence or be Taken Out. The same applies to it's attacks on her (since it does less damage but she lacks armor).
This would also Harry's comments about him being in the top tier of raw magical power would ring more true, since Harry can throw power 5 attacks around.
Actually he can throw Power 6 attacks around, maybe more like 7 by Small Favor, but what he's really referring to is raw Conviction...he has the
potential to be one of the most powerful.