Blighted Map - mob cannot die

There is a mob that cannot be killed - it is always a Rhoa.

The video that showcases this occurrence - https://youtu.be/DlfEP45JSi4

Basically, the Rhoa is being damaged by my character and all the tower types, but it is simply on 1% HP refusing to die.

This stops my Blight map from completion, as the Rhoa is stunned by the towers (there are ways around this, I am aware - go to another part of the map and hence disable towers, enabling the Rhoa to damage the pump).

This happens in every other Blight map that I do.