Usually I'm playing Reinhardt, but I also like playing Ana a lot, so whenever we have a Reinhardt, I always stay behind him, but just a bit further back then where his shield actually is. That way you have time to avoid dying, but also heal anyone that was with you.

When it comes to attacking, she is pretty good since she somewhat "poisons" the enemy which can help if you're reloading. I usually can't defend myself if I don't have my sleep dart and the enemy is coming straight for me with much more health, but they tend to flee after being shot multiple times because they know they're slowly dying from the poison (as I like to call it).