If you are talking about a level 4 NPC, then there are also requirements for Archery and Horse Back Riding Techs in order to take no losses. Age 1 and Age 2 are pretty equal on NPC farming. Also I have noticed level 4s are a bit odd and take a few extra ballista to be safe with very low level heroes. But if you have the right techs you should not lose an entire wave.

Cap