[InsainArcaneBirdbrain]

If number changes are hypothetically not an option then there's the idea of making it so you need to have Guard LV30 or higher for Mirrored to do the retaliation damage, but I'm honestly not entirely sure if that's slapping it too hard.

Requiring a minimum level for mirror to work is a good idea in my eyes, because it forces a commitment into that guard. Mirror enchant does so much as it currently stands as it can basically throw half of a melee attacker's damage back in their face for 1-2m, while still having enough momentum for the user to get 2 actions of their own. The lv30 guard means at least 3m has to be spent, which heavily restricts skill usage on turns you want mirror to work. As it currently stands, the effect is waaaaay too efficient for a potentially 1m action that punishes basic attacks (especially those which have on-hit damage effects) unfairly.