The Pandarens were trying to damage the Mecha Dragons with whatever they had. They saw how the dragons were able to regenerate the parts that had been destroyed or injured. In return, the Mecha Dragon would throw its breath at whatever humans it could find and sometimes sweep them away with its claws or tail.
But the part that scared them was not because these Mecha dragons were huge. Cultivators in Jin's dungeon store had already seen their share of scary huge monsters. What made them afraid was that when the Mecha Dragon seemed like it had enough of these pesky Pandarens poking its ass, their chest plates opened and small sized Mechanoids emerged from them.
