Super correctly would be to say: "you had better not do this". "You better not do this" sounds marginally ungrammatical, though it may technically not be.
Literally, it would be "don't do it better", but I think some additional info/word is needed for that context, or at least "nie rób tego lepiej"
Technically yes, but it would be easy to confuse. Just compare how often you need to say somebody each sentence and you'll see why. Maybe something like „Zrób to nie lepiej niż tak”?
I like it when we get these sentences that have numerous possible word orders and possible translations into English. It's fun. Definitely, in such examples it makes one realise just how difficult learning English can also be for Polish students.
How about: 'Better not to do this'. It may not be grammatical but many people choose to say this. If your sentence had Robisz not rób then I would agree your translation, but rób is imperative?
Yes, "rób" is imperative and therefore it translates simply to "do". Just like "Do it" is imperative.
"Better not to do this" means "It is inadvisable to do this", right? So it's "Lepiej tego nie robić".
You can easily imagine that there's nothing wrong about 'this' generally, but 'you' shouldn't do it. So in this sentence it's really too much.
In GB English, the "to" is too much, and jars, so "Better not do this" is better here ;-)
I was told that it doesn't really work in English, unless it's something regional...
Indeed. Some explanations - http://english.stackexchange.com/questions/10358/why-is-it-i-better-not-verb-instead-of-i-better-don-t-verb ; http://www.urbandictionary.com/define.php?term=You%20better%20don%27t