Auto-translated
If the save isn't accepted, then why are the rules allowing weakening if it's considered cheating?
1. On the map, there's a non-upgraded neutral creature. It either has an upgraded stack or it doesn't. This is determined at the beginning of the game. So, it doesn't appear only on the first attack; it's either always there or immediately absent... (We're not considering upgraded creatures, as the bug doesn't work there).
2. If there's no stack, there are no problems. We can attack these neutral creatures, use magic (or other means) to eliminate some units, and then retreat/lose. After this, the number of neutral creatures is reduced by exactly the number we destroyed. This is weakening – we CAN do this to allow a subsequent hero (on the same day or another day – it doesn't matter) to attack the neutral creatures and finish them off. It makes sense if we couldn't destroy the neutral creatures in one go. This is also true for guarding a resource or another object, where the guard is inside, not outside. :)
3. And now, the actual bug. If the neutral creature has an upgraded stack, after we retreat, all the remaining upgraded units will disappear. And the number will be reduced by the number we destroyed plus the disappeared upgraded units. For example, there were 100 Griffins, of which 34 were upgraded, and two stacks of 33 were not upgraded. We destroyed 15 non-upgraded units and retreated. As a result, there are 100-15-34=51 units left. On the next attack, some of these units will become upgraded again, for example, 17 Griffins (this is a feature of the cell where the neutral creature is located), and we can repeat the procedure – until we eliminate them all... :)
If we destroy the non-upgraded units and leave only the upgraded ones, the neutral creature will disappear from the map altogether.
This is the bug. It's prohibited. The scenario described in point 2 is weakening, but it's allowed. Find the difference. :)