Towards the end of chapter two you unlock a new area called "Arena." Here you can train your characters one of two different ways. The first way is to fight endless waves of enemies one level higher than you or you can send them off on a secret mission and get experience while you're away. Since the latter is based off your system clock time and has no "fast forward" prevention, you're free to send your character off, set your time well into the future and get a bunch of experience for doing basically nothing.
For those interested, it took me about 40 goes to go from level 15 to 99 and took me about half an hour to do it. For those looking for more modest numbers, you get about 3 levels until level 50 and then 2 from there to 90, with the last 9 levels coming one at a time. This will vary based off your luck, but is an easy way to hit your max level, keep unused characters relevant or simply get some quick AP (I had enough to unlock everything listed on my character with 11 to spare).
For XBOX ONE users:
I've gotten a couple comments about whether or not this worked on the Xbox One and how to do it. According to several it does work and Christopher Harris posted a nice little guide in the comments section, which you can find below.
"Hard rest your xbox only after putting ur xbox into offline mode (u do this through the settings) hard reset is just holding the power button down until it cuts off. Now after doing this the date/time are unlocked for use..and just move the year fwd a year and go right back into the game...it takes less than 45secs if ur quick enough to gain 2 levels.... it's faster to do this on the Xbox one than the ps4 because of the way the menus are set up..u can go right into settings from the game and then right back to the game"
![](https://i.ytimg.com/vi/VWg79ZmWVSY/maxresdefault.jpg)