Imagine this situation.

A forcemaster fights a priestress. The holy one has summoned a knight for protection. The forcemaster answers with a charm on the knight, so she can't be attacked. The knight, unable to attack, guards the zone with the priestress. The forcemasters goal is to attack the priestress.

Here is my problem concerning charm:
The knight guards the zone and therefore the priestress against enemy melee attacks, right? But the knight is charmed, so in his eyes the forcemaster, that attacks the priestress, is not his enemy. Why would he step in and place his body between them?

If charm is working as it should in my opinion, he could not step in, the forcemaster could execute his attack on the priestress.
If charm isn't working that way, guarding would be the easiest way to get rid of this amazing enchantment.