Obviously this “Guard bar“ mechanic if implemented it should also be a two-way thing.
When enemies have their shield up instead of taking reduced damage and you deal poise damage, they will have a Guard bar and if you reduce the guard bar to 0 they can no longer use the shield, and if you leave them alive for enough time they will recover the Guard bar and can use the shield again (obviously it is better to hit the enemies when they lower the shield), obviously when you hit an enemy with a shield up your attack “bounces” but I think it is appropriate to remove the poise damage since it is very strange to cover a shield and remain surprised for a long period of time (it is strange both when it happens to you and when it happens to enemies).