@Meddler: How would you calibrate a Champion with a shield-on-autoattack effect?

I just finished making a new Champion Concept with the following passive.

P – Code of Honor
Damaging enemy Champions with basic attacks or spells grants Celes a shield that absorbs up to 10 + (3 X level) (+0.15 AP) and lasts up to 1 second. Does not stack; refreshes duration and shield strength when reapplied.

For more context: http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/aGuGtOEQ-celes-the-hand-of-justice

The idea was to take Irelia's Hiten Style life-on-hit effect, retain the "Trade Superiority" it grants Irelia when she gets into an auto-attack fight with someone, but remove the sustainability it provides. Essentially, if you get into an auto-attack fight with Celes, she automatically has an advantage in that each blow she lands creates more "distance" between her health and yours. But if you poke her while her shield is down, she can't just auto-attack minions and top herself off; the damage she took will stick.

Making it balanced, however, is tricky, as I want this passive to scale into late-game and provide her with some nasty dueling power, but not make her indestructible. How would you go about it?