Is it possible to keep your companion from attacking right away?
Solution 1:
If you are in sneak mode, your companion will try to sneak too. This means that they won't go charging at the first enemy they see, as long as they remain undetected from that enemy.
Get a high enough stealth skill betwixt the two of you, and she'll stay by your side while you 1-shot monsters with expert sneak shots.
Solution 2:
Give her a bow and arrows. She'll shoot with you. You can also take away her weapon and she'll just use the bow.