MF is very good at doing quick dmg, every time she attacks a new target within a certain amount of time it does bonus dmg so make sure to last hit and when trying to push go back and forth with the minions. When her second part of the Q manages to hit an enemy it does a lot of dmg so take advantage of it when possible. Her passive resets when you q the target and it hits a second target, so if the enemy champs are lined up or you are pushing have at it.

In team fights wait for one of your cc champions on the team to line up people for your ult, and if the enemy team is charging get to a safe position so that your team can guard you while you ult your life out. Make sure to use your w before you ult because it slows which makes it harder to escape her ult(main source of dmg in a team fight). Once your ult is down stay just in attack range and auto the squishies, and if the cc/tanks are charging you attempt to kite. If you build her the way i prefer to you should be able to heal most of the dmg that they are doing to you.

Mf is very fast so she can run up to mid or even help with objectives when she pushes out which helps the team progress faster towards the win that everyone desires.

MF has absolutely no escape unless she hasn't been dealt dmg recently, so don't pick her into a mass cc team that will not let her have a chance to escape at all. She is very good at punishing champions that have a very short range without a gap closer/escape. Avoid picking her when the cc on your team separates the enemies.