"Can't anybody" is grammatically correct, but you will hear "can't nobody" in some dialects of English. As a second language learner, you should use the grammatically correct version.

Think of can't (cannot , can not) as negative. Nobody is also negative. we try to avoid double negatives. Same with all the No+(one, body, thing ...) words. Think how difficult it gets to understand with lots of Noes ... There is a famous line from an old animated film 'Bambi': "If you can't say something nice, don't say nothing at all".

"Nobody can hold us down!" = we are the best "Can't anyone hold us down?" = is there anyone who is good enough to beat us? "nobody"= not one person, "anybody"= is there one, or some, person?"

If you explain what you mean, maybe I can help you say it correctly.

"Can't anybody" is correct. I don't understand the rest of the sentence though.