I and one of my colleague were doubting wether this is a sentence or a phrase:
You can do better.
I am of the opinion that it is a simple sentence because it has a subject and a predicate. The subject there is "you'. The predicate there is "can do better".
With the above deduction, am of the view that "You can do better" is a full simple sentence and not a phrase. I don't know wether you will agree with that.