Player rank is the sum of the levels of all your different characters. They have to total 700 or more. Since the games work on a progressive leveling system, the XP needed for each level increases as the levels go up. Therefore you should switch around between characters to minimize your time spent.
You have 12 characters to start. If you buy the 2 season passes, you can get another 6 characters (3 per pack) for a total of 18, which would make it go faster (again, since lower levels go up faster). I'm not doing it, so it'll take longer, but the option is there.