There is one method that is effective I think you are all forgetting, it takes up a reasonable amount of space somewhere on your map as well as two (non-allied) computer player must be present, but it is as follows:
1] Set "Computer A" to be an enemy of "Computer B"
2] Place multiple reavers for "Computer A"
3] Nearby, place one turret for for "Computer B," surrounding it with invincible units so that scarabs are not able to acquire pathing to the units being attacked.
4] Use trigger "Give", as well as "Move," in conjunction. Move the Scarabs to a location centered around your hero unit, and they will spread randomly out and seek towards things.
I think you will be pleasantly suprised by how clean this effect turns out. And, as a sidenote, you can upgrade the Scarab damage.