Yeah, I kinda figured the king wouldn't be taken down so easily.
Anyways, this story has been pulling a lot of deus ex whatever for the past while now, so I'm really just waiting until the MC punches the king and copies his powers or he evolves all the skills he's stolen to date into some weird amalgam because we've already established that you can apparently do that and turn a skill into something completely deviated from it's original premise because why not?
I don't consider this a Deus Ex. The king's contract with a dragon and his near-invulnerability have been introduced earlier, so they don't come out of nowhere. The exact details were left intentionally vague, but we were basically told to expect him to survive this much. At least unless and until the insurgents pull out a dedicated trump card of their own to counter it.
The sage's "abyss mode" is closer to it, but I don't consider it as such either. Having a fallback skill in case the cube is destroyed or stolen was quite a predictable precaution.
I also do feel like the MC should find a more creative use of his ability to copy skills than just use them one after the other exactly as he has seen them used before. It gives him quite a variety of actions, but they ultimately have a predictable component. Not to mention how little training he has with each of them. Someone like the sage has fewer cards to play, but he's much more proficient with them.
(Something about "I fear not the man who has practiced ten thousand kicks once, but I fear the man who has practiced one kick ten thousand times.")
He does have the limit of being able to use a single skill at a time, apparently, but he could likely find a way to work around it. This would come either with practice or, more in line with the pacing of this manga, with a sudden flash of inspiration.