Freezing Field

[Expelliarmus]

But doesn't locust do that too?

No

And what if units start attacking the dummy units (but not kill because they expire too quickly)

If you give the ability Invulnerable (Neutral), it cannot be selected as an attack unit.

 

[MasterRofl]

All you can do is reduce collision size and take away its model. If you give it locust, isn't it untargettable?

 

[PooBucket]

Smart... but wouldn't Invulnerable... you can't target yourself? Oh, wait, can't you still target invulnerable people? In the targets allowed?

+rep Expel, you've got good ideas! ^^