SC2Mapster

On the spot i can think of one sure way.

Event:

Any unit enters X Region

Condition:

triggering unit is in units in region with alliance enemy to player X = true

Action:

Issue attack order on that unit

turn current trigger off

wait for condition: number of units in region = 0

turn current trigger on

Event: Any unit dies

Condition: triggering unit is in units in region with alliance enemy to player X = true

Action:

If then else: If: number of units in X region >= 1

Then: issue attack order on random unit in that region