This should be good.
So how the fight
could go. When the battle start, Subaru would need to enter Hope Peak, so he need to pass the
metal door, from outside, that have gautling guns waiting for him. That's not really problem because if he dies, he can comeback. After dealing with the guns by destroying them before they fire, he would need to deal with the army of Monokumas.
Thas't the hard part for Subaru and Beatrice.
First, Junko can control up to 100 of them at the same time, and all of them can just one-shot Subaru, Beatrice and Patrache. Second, In terms of combat skill, they complete darf Subaru team, and dodge all they attack with Analytical Prediction. Third, they can also self-destruct to take down all of them. Fouth, with this numbers they can totally overhelm Subaru and and make him tire himself out until he loses his stamina and mana. Fifth, Subaru can't really demage to Monouma. so he would need to rely on her spirit, and most of Beatrice attacks not gonna work against Monokuma, besides Al Shamak and Minya. Sixth, Monokuma is really good at talking shit, so he can rally Subaru.
For Subaru advantages in this fight, Beatrice can cast up to 40 magic arrows that stop time, so even if she hits them, a explosion not gonna occur, and he can take down most of them with her Danmaku. They also have a temporaly way to became invincible, and since Patrache is with them, they can easily escape for the other floors because she slight faster. And since Junko rely on her cameras to see what's going on, if they destroy them, they would render her vision a little. And lastly, Subaru is immortal so he can take all the time to finally reach out Junko.
So, it seems at first the Monokuma army would pretty much destroy Subaru at the first loops that took place, but little by little with his infinite retries, Subaru would eventually be able to win against Monokuma army, and reaches Junko. That's one of the ways Subaru could win. Even trought is gonna be hard and take forever, he could hold back all of them until he reaches the control room, and kill Junko.
So how Junko could win? Well, Junko is the Ultimate Analyst, and pretty much the smartest Dangan character, so after seeing Subaru for a while, she could figure it out eventually about Subaru RbD, and realize that killing Subaru is pointless, so she would use her other methods of winning besides killing. Since she is infinitely smarter than him, and Monokuma have higher LS, she can easily just restrain him, and drag him into room so she can do all the worst to him. She can social influence him to give the fight, she can use memory manipulation to make him forget, he can brainwash him with Acupuncture, she can use her videos to drive him to despair, etec;,,,