The green ray was so strong that it was able to push the Beetle King back even though it was charging at him at full speed. The ray continued to release from Mako's hand and the king was taking a serious amount of damage.
It kept getting pushed back and was completely unable to counter it. The top layer of the Beetle King's exoskeleton started to peel away as the ray was about to tear through a hole in its body.
Facing unbearable pain and an unknown ability, the Beetle King couldn't figure out its next move. If it allowed the ray to continue hitting him, he would for sure be dead, so it did whatever it needed to in order to survive.
As the Beetle King was dragged back by the ray, it moved past the regular Terra beetles on the way through and used its long legs to fling two in front of itself so that they would tank the ray for a little while.