I read this book when I was around 13 or 14. It was between 2015 and 2016. I remember it was the last book in the series and the back cover had an illustration of an (old) man wearing a green bowler hat and suit.
Basically, this group of teens, possibly three boys and two girls, were tasked to take down an evil sorceress. This sorceress had a daughter called Eris and she killed all her boyfriends. One of the female characters could turn into a Chinese dragon and was related to a villain from one of the first books.
I remember something about a golem taking the place of the main male protagonist and I also remember that the final battle took place on the Golden Gate Bridge in San Francisco. Additionally, the teens posted a spell on YouTube to try to find the remaining kids to fulfil the prophecy.
There were lots of elements involving time travel, knights (briefly), and hamburgers.