I remember dawn saying nothing could increase stun rate.
Stunning by non Magi (the plural for Mage) is not a Magic attack. It is a Skill attack. Magi stun using Magic. Non Magi stun by hitting really hard on the head of the opponent kind of thing.
The opinions expressed on this post are my own as a player, not as a Moderator.
tested around 20 fights with lvl 12 advantage and without advantage on bryn vs same player and couldnt see any difference to stuns
Crit has 0 (zero) effect on mages.
If you want to waste your rune slot for an Advantage, its up to you.