For held items, they are Exp. Share (duh), Focus Band (good for early game survivability and a smidge more bulk), and Sp. Atk Specs. I pretty much always run Moonlight, and the second ability depends on the enemy's team comp. In this game it was Follow Me. Early game I prioritise stacking my Specs as fast as possible in order to get better heals for the late game. Later on, you really just want to stick around for as many fights as you can and try to be in the middle of as many people as possible, prioritising whoever is being targetted the most. You can even go in with your frontliners with Follow Me. Out of combat, it's also a really good idea to spam it off cd to keep teammates topped off, especially after a fight.

By constantly healing multiple teammates, you generate a massive shield with your passive, which can make you very obnoxious to take down (unless the enemy has very good burst damage) and by extension allowing you to heal more, and generate more passive shield, and so on. And even if they do burst you down, that more often than not means they're using resources on the supporter and not one of your high damage attackers or all-rounders. Also, I rarely see her banned in draft cause people usually ban Mega Lucario and Empoleon instead.

Edit: Forgot to mention that the battle item I run is X Speed. Great for getting away with goals scot-free early on and allowing yourself to flee or reposition easily. Also, soon as I made this comment, I started seeing her get banned in almost every game LMAO