Last added unit to unit group?

I'm trying to find a way to have something like "Last created unit attack last added unit to unit group" is there someway to do this.

Every time you add a unit to the unit group, also have a unit variable. So say 3 units were added, you'd have:

Unit Group - Add Unit X
Set Unit X = Last Added Unit

Unit Group - Add Unit Y
Set Unit Y = Last Added Unit (group now contains X and Y, and Y is last added)

Unit Group - Add Unit Z
Set Unit Z = Last Added Unit (group now contains X, Y, and Z, and Z is last added)

Then your trigger would be ordering last created unit to attack the Unit Variable, 'Last Added Unit'.