In Aeria game there was a sense to stay level 19, cause there was a Janus range for 15-19 players. And some 19s were really OP at that time.
But now the battleground is 15-29. There is no sense any more to stay 19. As GoddessSand said, if you want just CX1 boss, you can make a char 27 to farm it, even alone. Every class can kill it, I remember I killed it a lot with my punisher when i was 27, and punisher is the weakest.