General consensus has often seemed to be that on the Boss for Valkurm you can just get unlucky & have it get off charmga, which can easily cause a wipe.
Thoughts around this also seemed to agree that killing Nantina either reduced the chances of the Mega Boss using Charmga, or severely reduced the duration of the charm.
My experience in clearing this zone numerous times but having wipes here & there would seem to back that up.
However, on a wipe the other night I noticed something I found potentially interesting, although it may just as easily have been horrific bad luck as well.
When fighting Nantina it did not get charm off at all, not once, it didn't even attempt to use it & get stunned. On fighting the Megaboss it did Charm 2 or 3x, and this completely screwed us.
My thought was is it possible that Charmga is only locked if Nantina actually uses the ability? I think I've only lost to the Megaboss here one other time due to Charmga, and I can't remember if Nantina used it that time or not. Can anyone think what their experience on that has been & if that holds any water at all?
Just a potential thought.