To rank up to level 11, you need to play rank matches, there's 2 ways to do this, legit online or find yourself a boosting partner.
There's something else you need to know, the ranking is different from the previous blazblue game, chronophantasma extend, the level is only for the character you chose, and need to play 10 match with it before actually start leveling up, for exemple:
Once I played 10 match with Terumi, my level went from AUTH to 7, then if I wanted to play with any other characters, the level was back to AUTH, so, find yourself a character you really enjoy and stick to it until you reach level 11.