There seems to be some dependence on the charge level. To make it last for a long time, you need to reach at least level 2, etc.
In this way, I eliminate deadly units – the box around the guards moves with them under Stone Skin, the guards don't take damage, they attack everyone who dared to hit their neighbor, and they deal return damage.
Added 4 minutes ago
It's cool, but you also need to accumulate points for this ability. In general, I think it's the best racial ability – it's the Humans' ability.
Added 17 minutes ago
If you first click Wait, and then cast Guardian on a unit, it will work when it's that unit's turn to attack – accordingly, the attack will be without a return hit – and in the next turn.