Not bad! Assuming that the fight's not done and it'll just hold you in an attack box indefinitely at some point, it has a reasonable difficulty level and looks pretty good!
Now a few personal hiccups: Checking Sans' stats, if this is post-attacks and his brother's the last one before the ruins, then Sans should have more than 1 ATK and DEF. I don't know the story here, however, so I wouldn't know where this fight is in your timeline. Papy could be the first victim, for all I know. Again, just a personal hiccup since I don't know the juicy lore~!
Secondly, not too much a fan of random-fire blasters. Can't learn anything if it's just gonna be entirely up to chance, y'know? It's not like those attacks were difficult however, so it'll probably be fine in the long run.
I'd include something blocking off the MERCY button! Probably nothing happening simply because of demo or your personal choice, but I think it'd just be a nice detail. No escape, no mercy.
And finally, just a temporary measure: something to tell us when the demo's over. Doesn't have to be anything big, just like an "END OF DEMO" or something in the final text box. Or you can have fun with it! Either way, just something to stop the future players from expecting more from the current build as it's somewhat dissatisfying for nothing to happen, y'know, y'know??
Regardless, good work! Looks like you're beginning to make something fun, so I hope you enjoy as much of the process as you can! Can't wait to see what you come up with!