You don't directly target anybody with the shield, you just press W and it detonate.
It will deal % damage against each target.
So, let's say: there is a 5k hp mundo, and a 2k hp vayne.
It will do (10%) 500 damage to mundo, and 200 damage to vayne.
The description is a bit confusing tho.
No, because you CAN'T select a target for it, you can just detonate it. I played sion sometimes tho.
Think about that: The shield detonates automatically, in that case, you are unable to select any target: How much damage it will deal?
Increasing depth if okay, but doing an 1500 damage aoe nuke if the enemy have a mundo is absolutely overpowered, no way riot would implement a thing like that. :P