It depends on what type of char Charlotte is on. If she's on an earth who is fast and can wall her, or on a water who can revive then it's possible to train her solo. If she's on a team then she will get hit less and die less so shell get more exp that way. Also if she can't one hit kill mobs on her own, a team helps with that too. If possible, try to make her the fastest in the team so she gets the most experience. Good luck.