Wasn´t it because he was provoked?

1. There are different "rotations" of the Doom Tower, with different bosses at all (for example, in the actual Doom Tower is no Frost Spider) and on different stages. Next month the Sacrab King may not sit in the Doom Tower or on another floor with another affinity. This month, all Scarab Kings you will meet (floor 20, 60, 100) are spirit affinity.
2. The Scarab King is not immune to turnmeter reduction (unlike some other bosses). Armiger is a very strong champ here (and in Spider and Fireknight, uncommen champ with legendary quality). Champs that can reduce the enemies turnmeter help a lot in this fight.
3. The Scarab King has a special shield, reducing all incoming damage. You can destroy that shield by reducing his max hp. If you have a champ in a destroy set or a champ that has the ability to reduce enemy max hp (the void rare Barbarian Scrapper is great vs. Scarab King), this will speed up the fight.
4. The boss will do a counter attack, hitting all of your champs, whenever he is attacked from a champ without shield buff. The strength of the shield doesn't matter, 1 point is enough, but there should be a shield buff. Champs that can provide shields to all team members help in this fight.
5. If you can't use any of the stragies above, because you don't have the champions yet, and you want to use UDK to slowly beat him down, the easiest solution for your problem would be doing the fight manually. If have no idea why your UDK behaves that way in this fight. But I guess you don't want to farm this boss, just doing the stage once. Investing the time once and manualing the fight would solve the problem.

/edit: If Wilson Theodoro is right and your UDK is provoked, manualing the fight wouldn't help.

Keep in mind that Doom Tower was released as end game content. If your account has only 2 lvl 60 champs at all, it's maybe just to early for you to pass this boss. Just go on playing, make more lvl 60 champs, farm better gear and beat the Scarab King next time.

Thank you for your quick reply. I will try what I can do in a manual fight. And yes I don't intend to farm DT yet. I wanted to see how far I can advance in DT at this point. But then I better concentrate on gathering and building up my champions. That's fine with me. :o) Tnanks again.