Isn't It Lovely

She was named the best demon tamer in all of LA at the age of twenty-two. She not only conquered each of the Psellus-classified demons but controlled twelve of them daily. Her strength allowed her to conquer over thirty demons at twenty, and it grows stronger every day.

Divenschia Ivicci and a man named Murdock Hecore are invited to tame the Devil himself after a deal was struck up. A legion of Fallen Angels have planned to destroy the world in a month's time, and the only hope at saving such a world is by taming and using the Devil against the legion.

However, the supernatural being is cheering her on and doing all he can to make sure she wins the competition. If not, then the Devil - deemed Daesyn by Divenschia - will be forced under government rule.

To keep him from being chained, all Divenschia has to do is beat the Devil before Murdock can.