The Scarab King is difficult to beat, as he has some unique mechanics that occure nowhere else in the game. If you want to beat the higher stages, where this boss will reappear, you should plan ahead, as it will be much harder for a solo Urogrim.
First of all the counterattack. The Scarab will perform that everytime he is attacked by a champ without shield. Solution: try to bring the rare (and booked) Lizardman Metal Shaper. He provides a shield lasting for two turns on a two turn cooldown. If that's the fastest champ of your team, the shield will be active all the time. There are other champs that can provide a shield, but Metal Shaper as a rare is easy to get and book. Maybe you have even one of them in your vault as food.
Another unique mechanic from the Scarab is his own shield, that protects him from damage. That's the reason why the damage he takes is so incredible low. You can destroy that shield with attacks that remove enemy max hp. A great champ for this fight is the void rare Barbarian champ Scrapper. Self-shield on all attacks and reducing the enemy max hp on his A2.
If you don't have any (good) champs that reduce enemy max hp, a full destroy set on a damage dealer will do the same.
