Crystal of Secrets

In Heroes of Might and Magic V, there's a bug related to the Crystal of Secrecy. Specifically, when placing these crystals, the AI sometimes disengages from the battle and allows itself to be attacked freely.
Of course, it's not good to exploit this bug, but sometimes circumstances force you to.
Usually, I place a stack of units, for example, Furies, in a corner of the arena, and then place two crystals near them so that they don't touch the units but block the path to them.
From my personal observations, I've noticed the following:
- The crystals don't work against infantry; Lava Dragons, for example, readily break through them.
- The crystals work intermittently with flying units. Sometimes they stand still, and sometimes they attack them.
Therefore, the question arises: is there a 100% reliable method or plan of action that, after placing the crystals, will ensure that the neutral units will stand still and not move?