Hi M.,
A Professor of mine once said a comment that has stayed with me throughout my career. "All behavior is meaningful, purposeful and can be understood".
Naturally, each individual is unique with qualities that make them who they are. Toddlers have not yet had the chance to polish these personality traits and the developmentally gap between individual toddlers can be quite great.
I would tend to agree with Rebecca, that your daughter though she may be more articulate could have a similar disposition to "lead" or "control" the group as does her young peer and they are butting heads over the "turf".
Obviously, it sounds as if your daugter is more sophisticated and so her interactions with her other classmates is smoother except for this one little girl with whom she clashes which leads to considerable frustration.
The other little girl also has her own motivation for her behavior which may be as simple as wanting to feel included but lacking the social skills to effect this goal and so she becomes frustrated.
As young as they are, it is not too soon to begin to teach our children the serenity prayer especially the part "accept the things I cannot change". None of us can change anyone else. We can only change our reaction to a situation.
Just like the person who dreads the commute to work each day because they are likely to be exposed to individuals who tromp on our rights and drive dangerously; we have to learn to accept that there will be individuals who wil really challenge our tempers, etc. but we can control how we react to them and subsequently our stress levels.
As long as there is no safety issues involved with this behavior, I would suggest you continue to communicate your concerns to the staff and let them handle it. Meanwhile I would explain to your daughter that "Missy" may be misbehaving for a lot of different reasons that have nothing to do with your daugter. Insist that she continue to respect "Missy" but emphasize to your daughter that if she continues to have conflict with "Missy", it may be best to avoid her at this time.
Preschools are an opportunity for children to learn social skills which they practice on each other. Missy likely will learn that her behavior is turning others off and begin to fine tune it especially with proper adult guidance.
Best Wishes,
J. L.