Okay lemme try -Impossible! -you kiddin' me! -Whaat!

No way! Seriously? Are you joking? Incredible! My goodness Good heavens It can be? I can’t believe it! Really? Are you kidding?

Okay so, I'm guessing you want to use slang and there are many ways that are NATURAL of expressing that. I'll just list them off I guess. "Oh really?" "Forreal?" ( Slang version of "For real?") "Is that so?" "You don't say?" "Why?" "No way?" "You're joking, right?" "Seriously?" It depends on the context of the phrase "How come" because it can be used in different situations. Many others, I have more, but those are really, really, slang terms, so yeah, if you have any more questions about slang, etc. i use it very often so you can take a look at my profile, we can talk about it, if you're down, hope I helped

I would just say "why". "Why" is neutral. It is neither formal nor informal.

Actually, "How come" is the informal way of saying "Why is that?" Jill: "I feel so thrilled and excited!" Paula: "Why is that?" Jill: "Because my parents said I could take online classes!"