When referring to something specific don't use "a". "Hello beautiful girl," is correct. You could say "Hello, a beautiful girl," but it would be similar to saying "Hello, girl who is anyone in a group of every beautiful girl." It still says the girl is beautiful, but now it is more like she's just a random choice from the group.

'Hello beautiful girl'.This is right but it sounds bizarre still... 'Hello beautiful' sounds better to me.

These other answers are all fantastic. Or you could try a one-liner that actually works.

Instead of beautiful use gorgeous.

I would say hello beautiful and omit 'girl'.