The Beetle King was gravely injured with a huge chunk of its right side completely crushed and obliterated from Energy Stream skill that was continuously adding up damage as it battled against the Lightning ray.
After it eventually won the power struggle, all that built-up damage struck the Beetle King at once, dealing massive amounts of damage.
The Beetle King had a tough exoskeleton that was only vulnerable to physical piercing attacks as a lot of force could be concentrated at a small point, thus greatly increasing the damage.
The brittle and hard exoskeleton of the Beetle King was very resistant to other forms of damage including the Energy Stream skill.
This was the same reason why Mako wasn't able to finish off that other Beetle King in the cave. Even though both of them were weaker back at the cave, the newly unlocked Energy Stream didn't possess penetrative properties and this allowed the Beetle King to withstand that damage and make its escape.
