How do I “Finish” the game?

I find parts of the game confusing. PS5 says I’m at 57% and I have 40% of the trophies, but I’ve done every store mission I can find except the kill the 13 brainiacs. I went to start the first one and was brought to a different earth or whatever, but there were 2 missions there and both said impossible odds or something. So how do I level up to beat them?

I also don’t know much about the live service stuff. Can I play old seasons? I also heard they only just added the finale to this game. Is that the kill the 13 brainiacs?

Lastly, looking though trophies it said start the Batman experience. How do I do that?