After experiencing it twice, the Cloud Serpent naturally knew that there was someone messing with it. They were using the power of the tribulation to weaken it, but there was nothing the Cloud Serpent could do.
If the Cloud Serpent divided its attention for a single second, it would immediately be burnt to a crisp and would never even have the chance to find out who did this to it.
So the only thing the Cloud Serpent could do was try to recover as fast as it could and try to break through this tribulation.
Based on how that unknown person was playing with it, the Cloud Serpent would be in danger when it came to its final bolt of tribulation lightning.
It was clear that the unknown person was using the tribulation lightning to weaken the Cloud Serpent before finally taking it out, so the last bolt of lightning would be the moment that unknown person struck.