I love how Bowser has character development in this series. He learned from the previous Mario & Luigi adventures and became a competent villain. He gained Antasma's trust and betrayed them before they could, and he would later be an actual threat for once.
You know, Antasma is a shockingly competent villain. He can put Mario to sleep to try and drain his life away inside of his own nightmare, and he's also safe from damage. Then he literally kidnaps Luigi mid battle and imprisons him in one of his licorice tootsie pops draining their dreamy teamwork powers, and forcing Mario to prioritize his brother. Real strategy is being used and it's amazingly competent.
To make one of his attacks being to literally force Mario to go to sleep and experience a damaging nightmare is by far one of the most genius things I've seen in a videogame.
This boss has to be one of the most memorable in the entire M & L series. Not because the fight itself is particularly noteworthy (though the fight is definitely excellent and does stand out from much of the crowd) but more so because of the setup. The twist of Bowser betraying Antasma was so unexpected, and so well executed. It was well set up, but not too obvious on the first time through, and it was totally in-character. But the best part about it is the unique scenario it puts Antasma in. After Bowser betrays him, Antamsa has nothing. No minions, no McGuffins, No nothing. He's just a somewhat powerful being. He has lost all hope of continuing his plan, he has no hope of defeating Bowser, and everyone wants him dead. He knows his chances of beating Mario and Luigi are almost 0%, but he has nothing left to lose except his own life. And nothing in the game points out this situation to the player; nothing that is, except the music. The Final Antasma Battle.
Remember that Eldream said that's it dangerous to fall asleep in a dream. This is what Antasma is making you do. If he won, I guarantee you that he would have KILLED Bowser.
You could just tell by the way that Antasma spoke that he was 100% planning to betray Bowser instead of ruling alongside him. He agreed with everything Bowser said and never spoke up when his suggestions got turned down by Bowser. If anything he was just playing along until he could finally do the betrayal. By the looks of it the battle was going to go down like the previous Bowser and Antasma battle where they enter the dream world and Bowser would absorb Antasma for extra power in the dream world like how Mario absorbs dreamy Luigi. But, since Antasma now has more power in the dream world which he showed off in the fight, he was most likely going to take over Bowser's body to use as a host and betray him like just about every other mario and luigi game. So I'm really glad that Bowser didn't end up being the punching bag of yet another betrayal, learned character development, and took charge for once.
Now that I look back at this, I think I realised something else about Paper Jam's final boss battle. Before the fight starts, paper Bowser and real world Bowser talk about how they're going to crush the plumbers then take over the world for themselves. Real world Bowser says he's going to take over the world for himself, which paper Bowser then looks back at him confused and claims that he thought they were going to make it their world, not just his, which real world Bowser rejects him and says no matter what happens it's going to be his world only. So Bowser has officially learned how to avoid getting backstabbed by villains working alongside him. Although he agreed to fight with paper bowser, he doesn't put his full trust in him and decides to go with the safer route of just betraying the other person before they get a chance to betray him. Since paper bowser has never had this experience with learning from his mistakes, he's confused because he thought they had a partnership that they agreed to keep. They decide to hold off the argument until after they destroy the plumbers, which unfortunately for them, they never got a chance to finish that argument :(