Okay, time to back something up. Beerus is not weaker than Demigra, even when Demigra transforms. The proof is in our main character, the only way to beat a God Ki Infused character like Beerus, Whis, Champa, Vados, ect. Is to have God Ki yourself. Yet, our character STILL beat him, also Demigra's power was felt by everyone, even Time Patrol Trunks, who understands NOTHING about God Ki. And with the old Toriyama power scale, (That was made before the Revival of F movies was even a thought) being retconned, the whole SSJ3 god infused Goku thing is no longer a valid argument. With God Ki, not only is your defense heightened beyond belief against non God Ki users, but with it's Regenerationn, and how the gods actually infused it in them to pratically make them immortal in combat, not by age, Demigra would have no chance against Beerus. Nothing, nada.
However, all that being said, let's get down to Hit and Demigra. First off, Demigra does have many abilities, being a Majin, such as having certain dimension bending/time bending/space bending abilities. If it came down to pure power, he'd be the power of eight Super Saiyan God Goku's, but that's on base. Transforming, it seems as if he's around 40 Super Saiyan God Goku's, remembering the life and power change from the game. That's almost as powerful as a normal SSJ from the Revival of F saga. Hit however, is far beyond that. Being able to take on a Super Saiyan Blue Vegeta and Goku, right after the other, (Even being able to knock their healing factors away, since those two aren't truly infused like the real gods.) and beat them sort of efforlessly. We don't know how much stronger Super Saiyan Blue is from a normal Super Saiyan Beyond god, but considering how SSB Vegeta one shotted SSJ Cabba (As strong as a Super Saiyan Beyond god.), I'd have to say Hit has Demigra completely. Demigra would only be able to match up 4/5 of a Super Saiyan Beyond god.
Okay, so when it comes to power, automatically Hit has the advantage, what about abilities? Well, we don't know too much about the ultimate assassin yet, but we do know some about Demigra. Demigra is a Majin, he's been alive for millions upon millions of years, trapped in the cracks of time. From in there, he can control beings as powerful as Frieza, Cell, and Majin Buu. As well as giving them a bit of a power up. He can travel through dimensions at will, time, and with some of the strange magic he's got, can even bend reality to an extent. If he can get his spell on you, you WILL be his puppet, and when he doesn't need you, he'll drain all of your power and instead of putting it into the cracks of time, he'll fill himself with it. When he's free but not transformed, he has the power to control Ultimate Gohan, Super Saiyan Three Gotenks (As well as keep them fused.) and Super Saiyan Two Vegeta, while controlling Frieza, Cell and Buu all at the same time, as well as giving them all an enormous power boost. When he is transformed, he has the ability to put reality itself into danger. Which is pretty much tearing a hole through everything, and letting reality collapse in on itself. And in death, he can still control beings such as Baby (Great Ape), Super Seventeen, and Omega Shenron at the same time while giving them a power boost. When it comes down to it, even if Demigra can't beat the true gods of Dragonball, he's still an extremely power and dangerous opponent.
And now with Hit, we don't know all of Hit's techniques or abilities, Hit is also indicated to be much younger than Demigra, so more than likely he doesn't really have a technique advantage, but he does have an experience advantage. An assassination experience advantage, while Demigra was biding his time, Hit was going out and killing who knows how many opponent? Hit's most noticiable technique is the Tokitobashi, or the time leap. Basically he stops all of time for .2 seconds, and for a being who move much faster than light, making light look like Snorlax going uphill, it's a very useful move. So useful, we'll come back to it later. Hit's technique is to go for the vital areas, and he even has knowledge of the vital areas of other species, such as the Frieza Clan, when he hits them, he concentrates all his ki through his knuckle and BAM! It goes straight through your vital parts, and comes out you back, smaller than a microscopic level, but so much of it is being shot, that it looks like sparkle or a beam come out of your body. He needs less time than instantaneous speed to pull it off, to make it more deadly, he combines it with the Tokitobashi, making it a technique to fear. Even with all this, EVEN WITH ALL THIS... His most powerful ability is his "Improvement." We all know how Shonen characters are, they get up, make some speech, scream, power up, and bam, they're stronger. Well, Hit doesn't need to do that. He only charged up in his fight with SSB Goku to make fun of how ridiculous it was. All he needs to do, is size you up, understand your... Bam! He's already improved before I could even finish my sentence. It's so quick, so dangerous, and worse, he improves to the point that he can kill you. Getting used to your techniques and abilities, transformations, ect. Ect. In fact, with his battle against SSB Goku at Kaioken X10, not only did he improve in the middle of a Kamehameha, but it improved his power and Tokitobashi so much, that he stopped time, within time. Wait... WHAT!? That's right, in the middle of the Kamehameha, as Goku is coming to knock the "H" out of Hit, he stops time, but Goku's still coming, because at this level, Goku can break time. So he improves it again, making it so he can put an extra layer of time stop on Goku so he can get away from the combination of the punch and the explosion from the wave of energy. And what's even scarier, even when improving, he still held back a ton, so as not to accidentally kill Goku, becuase Hit can knock the OP God Ki off of temporary gods such as the Blue form. Hit, in the end is truly one of the best assassins in any fictional universe, being able to improve to the point he can break physics. But the question is... Who would win between Demon God Demigra VS Universe 6's Best Assassin Hit?
Well, in the end, it would probably be Hit. While Demigra's age and abilities far outdo Hit, his abilities are a bit on the slow side. I mean, Demigra was meant to be in an RPG, hiding behind everyone while he slowly tortured the other team, not try to be to next Frieza. Why hiding? Because powerful spells don't matter if you can't land them! Hit is not only extremely agile, but can stop time. And let's say somehow Demigra hits him with the mind control spell, then what? Hit would improve to the point the spell would no longer affect him. And to make this more of an embarrassment on Demigra, his "Demon God" form is ACTUALLY SLOWER. "Agh! Fear the universal destroying blue lizard as he slowly floats towards me and slowly slashes at me!" Really bruh? His spells may be quicker and on a larger scale when transformed, but they're predictable! Hit's not an idiot, he's a very adaptive fight, so if your spells are predictable to me, a guy playing his PS4 just trying to unlock all the characters, they'll definitely be predictable to the quick, time leaping, self improving, awesome skin color of purple, Hit.
Power-Hit
Speed-Hit
Techniques-Demigra
Durability-Hit
Abilities-Tie
Experience-Hit
In the end, it would be a fun fight to watch, but it is a fairly predictable match.