If it's a situation where safety is concerned (i.e. she is about to run into a street), then you need to physically restrain her. If that means picking her up kicking and screaming and dumping her into her room (or car seat, if you are out and about), well then, so be it. You're bigger, you're stronger, and safety is not negotiable.
When it comes to transition times (for instance, she is coloring and it's time to stop and eat lunch), have you tried giving her advance warning? For instance, "Mathilda, in 5 minutes, we're going to stop coloring and eat lunch." and then 2 minutes later, "Mathilda, in 3 minutes, we're going to stop coloring and eat lunch." And then 3 minutes later, "Mathilda, our 5 minutes is up! Time to stop coloring and eat lunch." Then start to put the crayons away. Both of my girls are strong-willed to say the least, but this trick helped us. Somehow having a few minutes to adjust makes it less of a battle. Additionally, never, ever say, "Mathilda, do you want to stop coloring and eat lunch now?" Use, "It's time to..." so you are not asking a question. Asking a question gives them the chance to say no, and that sets up a power struggle. Remember, kids need a leader - be the leader.
Our youngest is hugely melodramatic (cries about every little disappointment) and when she starts in with that, I tell her, "You'll have to do that in your room until you have control of yourself." She has every right to cry, and I have every right to confine this behavior to a different part of the house than where I am.
Lastly, I leave you with John Rosemond's Bill of Rights for Children: http://www.rosemond.com/--RosemondsbrBill-of-Rights-for-C...
This is our "bible" of child-raising. Straightforward and simple and it works.
Best of luck! And remember, a strong-willed child is usually a very bright child. That will be a very good thing long-term!