As far as I can tell, Flash is essentially treated as a 600 potency attack that cannot crit, scales in the same way all of your other attacks do, is unaffected by FoF because its magical, and has a special secondary effect of doing no damage. This means that under Shield Oath flash first has its power reduced to 80% but its enmity generation doubled. Since Flash does no damage anyway this essentially means it does 60% more enmity under Shield Oath, just like all of your other attacks.