This is OP.
Your attempt to add a drawback was pretty much pointless since most of the creatures aren't going to have a chance to hit you.
Even worse with the upgraded one. Why don't you just add a part somewhere in the notes that says "This card is a permanent dim shield"?
Further more, you wasted card space by saying physical attacks. All shields apply to physical attacks only unless specified otherwise. An example would be adding the word "reflective", which would make your card reflect spell damage along with whatever other effects it may have.
So while you could of added something else, you wasted card space.
75% is already very powerful. 87.5% is almost a guaranteed miss. That means amongst eight creatures on the field, theoretically, only one of them will hit the enemy.
Nerf it.