After about 30 or 40 tries my sister did manage to beat both Joseph and Seb. Mostly used Rockets and Dynamite. What she did was wait behind the bus near the elevators, hit them with a rocket as soon as she regained control of The Keeper, spammed rockets until she ran out, Joseph didn't even regain his footing, then used the dynamite to finish Sebastian.
Cheap, but it worked. She's also done it without getting hit, saved each time she faced a boss and used just the Rocket Launcher to keep her distance. Much easier than the final chamber!
I'm not fully thrilled with it. The storyline is kind of cheap and a little confusing to me, even though it's straightforward. Seems like Mobius used the dad as a test subject, but it doesn't explain why he appears as The Keeper, other than they want him to I guess. But if that's the case, then you'd think they would have been able to stop Ruvik sooner.
But the two biggest gripes I had are not even that big. I hate Zehn, the Sadist, RPG Sadist, and Amalgam having physical bodies. The Sadist, yes, he was a murderer hooked to STEM, I get that. RPG Sadist is now a separate guy instead of just The Sadist picking up a new toy. Zehn, and Neun who gets the short end of the stick of that duo just like Joseph I guess, are now not only totally different at least backstory wise, but Neun is dead? I always had the impression that what happened to Neun and Zehn happened outside of STEM and they were made to be guardians of the church like Sentinel. Granted, I just got that from the notes about them, and this might be the real deal, since there is very little about them anyway. And Amalgam is supposed to be just that, an amalgam of the personalities and people in STEM. Granted, you could easily counter all of that with "non-canon!" But it bugged me.
Also, the reuse of the Water Tank Room three times. Joseph could have had the room you fight him with in The Assignment. Seb could have had the Water Tank Room to himself, or perhaps the room in the city when he turns momentarily. RPG Sadist could have had the gauntlet area in Ruvik's mind at the end of the game when he appeared.
But in all, pretty fun, really great to play to waste time or vent some anger, but a big let down for me.