The Power of Six by Pittacus Lore (2011) HarperCollins

The saga of the Loric Garde continues.  Those who came to Earth with their Cepans (guardians) in a desperate attempt to develop their Legacy powers, learn to use the tools in their chest (which only opens the hands of both the Garde and their Cepan, unless the Cepan is dead), train and take back their planet – while evading the Mogadorians who have come to Earth to find and kill them.

A ‘charm’ had been in place to keep the Mogadorians from just killing their targets in any order.  They have to find each successive numbered Garde or the attempt will backfire and kill the killer.  But now, Number Six has found Number Four and the charm will no longer work, putting all the Garde in danger.  Number Six and her Cepan were captured by the Mogadorians and held prisoner.  While there, her Cepan is killed in front of her.  Back in her cell, she develops a Legacy power of invisibility and uses that to confuse her guards who rush into her cell and she escapes.  The Mogadorians still have her chest, but she develops additional powers, including the ability to control the elements.  Between them, and Number Four’s friend, Sam, they begin to defeat the Mogadorians each time the catch up to them.  Clues lead them to understand that Sam’s dad was an ally and hid something at their property under a sundial.  This takes them so close to Number Four’s girlfriend

But they are not all-powerful.