I'm not at home right now, so I'm afraid of not being able to help you out correctly.
Well, first, try to use only one dummy unit, ordering it to cast banish on all units using a unit group. E.g.: Unit Group (enumerate all units/targets) -> move dummy unit on top of picked unit -> order it to cast banish on them. IIRC, If you make a dummy unit unable of turning by setting its movement speed to 0, reduce both of its animation - cast point and backswing to 0., they become capable of casting an ability multiple times at once. If this doesn't work well for you, then create a dummy unit for each target.
About the movement speed reduction, if banish does not have any field related to it, then I guess you'll have to change that value in the gameplay constants.
(WE -> Toolbar - advanced -> gameplay constants...)