How about giving some unit an effect immediately when it is trained/built? Like first 20 seconds unit could receive movement bonus. Or creating mercenary units with limited timespan. Firstly giving it natural -10000 HP regen (I just picked some random big number which would insta-kill it) and then give +10000 HP regen as effect for, let's say, 20 seconds. So the Mercenary would be dead in 20 seconds. Currently I have done my mercenary unit so that it just starts with negative hp, but better would be expiration based on time.