Merged Iron Skin + Mirror Shell into one THEN create a new one
Both is situational but worst choice than conditioning at most time.
THEREFORE I suggest you to create a new rune that is bonus for shield based champion like this
> shields her/himself for 5% maximum health, refreshing if he has not taken damage in the last 10 seconds.
OR
> If you take 200 (based on level) damage within 5 seconds, you gain a shield for 5 seconds equal to 25% bonus health that decays after 0.5 seconds
The number is just idea... but not meant to be too strong.