It's a cool rune but I find it weird that the champs, without considering items and keystones, 
can't realistically use the rune despite physically bashing enemies with their shields.
Didn't consider 
. I'd call it a buckler but I guess you can call it a shield as well.
...this is actually hilarious. Of the champions with shields (Braum, Leona, Singed, Pantheon, Poppy) only one can benefit from this rune by default.
I think I have a solution.
Just code the different defensive spells they have as being "Shielded".
So when Braum uses Unbreakable (his E), he counts as Shielded.
When Leona uses Eclipse (her W), she counts as Shielded.
When Pantheon has Aegis Protection (his Passive) up, he counts as Shielded.
Singed... Singed I couldn't find a satisfying solution for.
Maybe as long as Insanity Potion (his R) is active he counts as Shielded?
To make this work, all they have to do is give a 1 hit point Shield when activated that constantly refreshes when it's broken.
They could make it more 'tank exclusive' by having it scale off bonus armor and magic resist. It'd more exclusive to tanks or champions that do build those bonuses rather than anyone who can tack on a shield. It would even synergize well with the Conditioning rune. It would make even more sense in that aspect since Leona and Braum gain bonus magic resist and armor when they use their W and E respectively.
But it is an interesting feature to have I'll say. I know of Slay the Spire, a card for the warrior called bash, which does more or less the same thing. Use 'block' or 'shield' points as a means of attack points.
In this case champions that build armor and magic resist can use it offensively periodically, and give their basic attacks that extra 'oomph' and 'thock!' for doing a rotation of abilities rather than champions like 
who get fat shields for going in, when weaving an auto attack, just gets more blatant damage to their already existing high damage output. And they're not even building tanky, they just have fat shields.
Yo let's talk about 
shield bash at 2hp and a lifebar full of shields and full mana 
???
Okay but why would you want to use shield bash on leona/braum when the "enemies you cc heal your teammates when they hit them" rune is right next to that rune.
Braum specifically benefits greatly from having allies hitting the guy you tagged with Q since it helps stack the stun up faster.
