This bug is called Multidefeat! It is strictly prohibited in offline mode, but what about online? This is a very important question, and it often tempts me to use it! :D
Let me explain a little - I didn't run away from the battlefield; I completely buried my hero along with his entire army!
A bug is when you run away and don't lose anything. But if someone likes to lose their entire army along with their hero, they can use this bug:smile12:
And to avoid killing an upgraded stack, you have to try (unless someone doesn't fight in melee) and kill all the non-upgraded units. As for splitting the neutrals into two stacks, one upgraded and one not, you need to have a large army for that, and I think it will simply kill the neutrals!