How to defeat the Beetle like thing in the extra mission?

I believe you're refering to Belphegor at the end of the elavator of doom.
Turn to Minotaur, start hitting it and eventually it will get mad and roll towards you, use Iron Guard and it will knock itself out for a while. While it's knocked out change to Wendigo, grab it, and throw it into the water then follow it in the water with Merman and begin shooting it with water. Repeat this and It should die fast.
I hope this helps.