Suppose we had a shield line that worked like the monster shields. They are invincible within a limited arc (to be defined), but you cannot turn or move while they are up. They also have a minimum "up-time," during which they cannot be lowered (also to be defined). You can choose to hold them up for longer, and then you can lower them at will, but not before the minimum up-time has passed.
3*: 45-degree frontal arc, 1.5 sec up-time.
4*: 90-degree frontal arc, 1.0 sec up-time.
5*: 180-degree frontal arc, 0.5 sec up-time.
I have a hunch that the 5* is overpowered. Got any pointers for me?
You have the up-time, how long is the down time? I think that'll determine wether they're OP or useless.