What does 'oye amigo dale cuello a ése' means?

"Oye amigo" means "Hey buddy/friend(either works)". "Dale cuello" Directly translates to "Give him neck" but thats not what the expresion means. That expression usually means to kill, murder, basically giving someone neck is like slang for killing them. "a ése" means "that guy'"

So in brief, "oye amigo, dale cuello a ése" is "Hey buddy, kill that guy"