The best I got is to try and troll him by constantly trying to tell him ways his antics could be better. By pissing him off, he will loss my mind game. thus turning the tables and assuring that I at least won something. So long as I can keep one upping his ideas, he losses.
I actually had a similar dream, here is what it was and what I did. So I am in a festival and there is some girl watching me. I run, now I am in a mall and the same girl is still watching me. I run again, I dont know where and shes there again, literally passing through people and she mockingly says to me 'You can never get away from me no matter where you run to' A big no-no is to mock me, that stuff just fires me up to unimginable heights, and to unimagineable heights I went to. So I looked for a hot air balloon, and as I was going up the sky, I looked at her below, she was angry from how I saw it. I was mocking her all the way. So the demon? Since we are playing a game, and he has already killed someone I care about. play a game I will. But what is a game if there are no equal stakes? After having all these powers, the demon only dares to watch? Since he had been making all the games until now, it would only be fair if I made one up for him now. I will then make the demon turn himself into food, I will proceed to eat him, then shit him out, he can then regenerate from my shit. Oh we are not done, not so soon. Everything has its bane, thats just the way things work. I am going to find what sticks to him, maybe even holding my urge to shit, where will he regenerate? Immortality or no immortality, pissing me off will be its worst mistake. I dont care how many it killed before, but when he chose not to kill me, he sealed his fate. No, maybe after he killed me, I would go to hell and then play with its family in there. When he gets back to hell, he will see his family in some very dire state. Now I am myself a demon, hello demon, you are not getting away anytime soon....
In a Danish accent** "he is clearly a troll." And if I learned anything from portal, you don't feed trolls. Just do not respond to him. You can play the game but if you show no reaction he will get bored. And if I learned anything from Rick and Morty, the world is meaningless and it nihilistic. As such if a being is so powerful as you have described then he clearly has nothing better to do, is depressed and wants to die. Oooooor he is not so powerful in perspective and has a more powerful bigger boss or God over hi and he is taking his stress out on you since like you he is in the submissive position getting his ass f*cked by someone he is helpless against. **Continues eating my sandwich Thing tjang thing Lucy lai.
If he is constantly reading your mind than at least you could sing in your mind all the time one of those stupid and agravating disney songs that seems to never be able to get rid of... You can also think of worthless, useless and sensless stuff, meaningless questions and pointless half complete sentence, remember agravating noises, flustering peoples and things that you encountered in your life plus all the annoying stuff you can imagine all the time without ever stopping. If he want to make you live in a living hell while percepting all your thoughs, you can at least make sure he doesn't have a perfectly enjoyable time, heck it could even make his concentration slip or say something he shouldn't... After all you have ten years to use the fact that his mind is open to your thoughs...
Not sure if I would even bother with trying to survive in such a hopeless situation, but I would probably ask him out of curiosity if he have any other hobbies, if he spent hundreds of thousands of years just torturing/killing people of course he will be bored.
hmmm, I think I need more details in order to facilitate a solution to your problem. pictures might help too
Just a guess based on it's power set and the location I'll guess the demon will use a psychological and physical test (that's what all test are about) like death parade so your choices and ability will determine your outcome if you take note that it will cheat (it's a demon)
What if your sister is the demon? Yeah, i'll probally be the type that gives no fks. Torture? Sure, i lived my life in hell. Family? Hell, all my family are demons. I think the demon would only watch my everyday life and he'll already be satisfied af.
It's easy! Just use a silver bullet! Oh wait... that's for werewolves. Then expose him to the sun! Hmm... that's for Vampire. Bash his head? Right... it only works on zombies. Uh... Poke its eyes till it explodes? ...Yeah, that only works on monsters in Zelda. I guess I'll just curl up and cry. No wait! Use a Giant Devil Robots to kill the Normal regular ol' plain Devil! Since regular ol' plain human can be killed easily by their robot counterparts, then a devil can also be easily killed as well! I'm a genius!
As it was me who summoned him in the first place (heritage!!!) ... I tell him to screw off or I'll just ban him with my mighty ban hammer (supplied by my part-time employer: Ordo Malleus) while doing a wiggle to the left, wiggle to the right dance.
Corrupt the demon! Demons are prideful. Challenge his power! Say he isn't all that powerful and have him show off said power. Turn this aganist him! Say he isn't all powerful if he can't transform at will. Genderbend him! Rush him and rape him! Er her! Demon corrupted! Your own body/soul is corrupted! You have demonic powers now. Continue to make said demon your own personal bitch. This is cliche as F... Might need to tap into some Fujoshi power or something... not sure how to do this. Say you plan to rape a male demon. Borrow Fujoshi power to do so. Genderbend said male demon first? Or during? Might be best to do so during to avoid repercussions for contractual breach...
Start a therapy sesh and explain to him y he is doing the things he does. Then follow up with the traumatic experiences he shared with me. Afterwards, I start a bromance with him and we conquer the world together. Totes not in a gay way.
"If you're wondering what powers he has, just think of the classic demon. He looks like an average human with a constant grin. He has telekinesis, telepathy, teleportation, pyromancy, immortality, healing, able to resurrect the dead, ability to create pocket dimensions and so on." God damn. Too much OPness! If there's no other way I might as well strike the heart of my sister with a knife instead of her being tortured to death. (I'll make sure that demon will not be able to resurrect her) I'll make him use all of his skills and observe how they work, from CD, Mana consumption, Stamina... everything and make countermeasures that will surely kill him in one go. Then, after killing him I'll eat his flesh and become OP, and find ways to resurrect my dead sister into anything until I have the ability to make her own body.