I came back to running sa2b after a too long brake, and I want to practice curtain bosses ie. egg golem eggman2 shadow2 but don't want to go to all the other bosses, just for the practice, is there a way to practice individual boss levels?

I've started running this game like a week ago and now I accidentally found a way to get OOB in CE, here's a link showing what happened. Can someone explain to me what happened, and see if this can be useful?