You could..
Trigger:
- AutoKill
- Events
- Time - Elapsed game time is 5.00 seconds
- Conditions
- (Unit-type of (Triggering unit)) Equal to Footman
- Actions
- Unit Group - Pick every unit in (Units in pink <gen>) and do (Unit - Kill (Triggering unit))
Replace "Footmen" with your unit
and "pink <gen> with your region
Ta Da!