First there is some problems here. Question really needs improvement I say.
Power and size of nobility
10 times stronger is really good. But how big are they? We talking about several thousands people? Couple hundred? 30 members of a royal family?
Because even with the increased power you can still just throw 10 times more people to get them. The problem is usually logistical as how to support those more people.
But war is not just won by pure numbers or strength.
I'm certain an army of a 100 nobles can be defeated. But an army of a 100 nobles also supported by thousands of other normal soldiers is a lot more dangerous.
So. Your people can still be killed, they can still be captured, just much harder.
Basically why won't 10000 people overpower just a royal family of 5? If you answer because the royal family has their systems of government including spies and soldiers, then them being super has little to do with it. It's just an advantage, but not the only tool.
We really need this question to be more specific.
Medieval Europe was not one place, nor did it just have slaves and masters.
Big continent with a lot of kingdoms and loyalties and alliances and all that.
This means that this idea of: Bad nobility poor peasant is very unlikely.
Unless in your story all nobility are naturally inclined towards evil.
Unless that. Then it's sure as heck you gonna have some good kingdoms and good nobility.
Also actual nobility did not just exist in a vacuum. They had a structure to society. And unlike what the movies say a lot of it was good and functioned. Anyway by that I mean you gonna have a lot of classes. Merchants who made money by trading, you gonna have soldiers. And of course a minor nobility, clerks, religious workers, knights...etc.
All those classes were part of society and worked with both sides.
Say a knight is given a small piece of land, he has tenants. He also needs to provide for his lord, but also for his people. He can't simply murder and rape and pillage as a valid way of life. So. He has to satisfy both sides. Even without courts. If you ask too much people will simply fail to provide. And a knight is a link in the chain.
His lord has much more lands. But those lands are not some NPC populated areas. They have people. Work the people too hard and your mines, farms, towns...etc are deserted. So. Again. In order to continue ruling in peace you have to satisfy both your superiors, king in this case, and those under you. I recommend The Prince by Machiavelli as an excellent read.
As absurd as it seems to tell you: Having magical people rule won't change things.
I actually stand by it.
Think of what we had in actual history. Aside from security kings should remain largely the same. You still have to convince the people to fight, you still have to rally the nobility and promise them stuff, you still have to claim to be in the right. The increased power is amazing, but not enough to God-Emperor the world.
And you are opposed by other nobility with the same power.
So. What really happens is that things don't change much. Sure. Abuse peasant only to have them flock to another king. Also it does not take a genius to figure out what you can use internal strife as a way to undermine a kingdom. Or a son rallying the people to take over the throne from the father under the pretense of justice.
Even nobility with power is not stupid. You can't go full psycho mode without some serious issues and opposition, from inside and outside. The most likely scenario is a family member taking over because they know that achieving a 100% despised rating with every single thing outside the royal palace is really bad for the whole family.
What do you do with the kids?
250 years is a long time. I mean Queen Elizabeth II notwithstanding. With 250 years of age the amount of kids a king and queen can produce is a lot... a lot lot. Sure. Human concubines are not producing anything magical. And even with just 1 wife. You still having a lot of kids. So. What do you do with them?
This is the biggest issue. Even if the ruling king decides to have less children, fine. But what about another branch. His brother might just think: If I just take it seriously I can literally produce a 1000 super soldiers in my life time. With some minimal planning he can just take over. He is still royalty. And his brother king only got 3 kids.
Kinda like Targaryen civil wars, except with humans instead of dragons.
Same with other branches working together. This I think is when things are interesting. When the individuals have power and marry it with political savvy. Then you can have good stories.
Just tune it down
You can still have interesting stories with that. But don't fall into the trap of: Evil kings wakes up and murders 100 peasants for fun.
Having actually powerful nobility and living to that age can be really good. From obviously noble family A which is good to noble family B which is bad. To the effects of long age on humans. To armies of noble people fighting and so on.