To get this result the attacking army must have a negative attack value.  The whole army must be negative, not just the leader.  To do this the wrong type of armor, weapon and mount must be equipped on the leader and the troops.  In this instance the equipment was plains spear and plains armor used against a force on a small hill.  Plains spear on hill is -24%, plains armor on hill is -24%.  Total -48%.  Doubled for an elite army.  -96%.  They had done this before.  What they had not done was add Elven Thoroughbreds to speed the cavalry elite in time to hit our siege before Timex was razed.  Thoroughbreds speed any unit by 15% but the add a negative 20% penalty to any non Elven unit.  With these mounts added the human cavalry elite now had a -136% attack value and I got a very puzzling combat report.  It was not until we harvested the gear that was dropped and saw that it was the wrong terrain type that we had a good working theory.  Then it took another day to test it.

 This is not a recipe for infinite troop creation.  It has to be elites and it only works on one side.  If both the attacker and the defender have negative attack and negative defense values only one wins.

 I am sure there are some interesting variations that can be done with this but I am too lazy to test them all.  Best if the Dev's just fix this one.  One thing it can't do is create more harvesters.  A negative attack army still kills harvesters.

 - Lagavulin