By the end of the event. You can reach it in couple of hours with current half AP+double EXP in Asteria anyway. You pick a battle, press auto and look from time to time to skip a cutscene and press auto again.
The only thing you need is a device (hopefully your phone) with Tales of Asteria installed. During the event, there is a dedicated page for "linking" your ToA account (no registration needed) to your GBF account. When you press that "link" button, it attempts to open your ToA game and automatically ties those two accounts together.
Good thing I can multitask while playing Asteria, because it SUCKS. Once I hit 30 and get Milla, it's getting deleted.
There's a second Tales event starting at the tail end of the first one. If there is a second character that requires ranking in Asteria (like with the IM@S collabs) then you'll probably have to hit 40 or 45. :<