To unlock Charge It Up you must defeat 100 enemies using Charged Shot.
Firstly, you must unlock CS from the merchant.
There are four kinds of charged shot, one for each elemental charm. You must have found the elemental charm to be able to unlock that shot.
Now, when firing your bow, hold RT and you will start to charge. When Meredith glows, then the charge is complete. Release RT while holding the direction of aim on RS, and you will fire a special multiple arrow attack.
Simply kill 100 enemies using these charged aoe attacks.