Recruitment
By fighting in the arena normally, chances are you won't be able to win, given your choices of monsters. The trick to winning - and therefore getting Janice to join you - involves making use of Sprigg. Equipping her with the Forget-Me-Not Pot (found in Chronopolis) allows her to add more monsters to her transformation list, providing she delivers the final attack against them. By gaining more monsters, they also become available in the arena. Get strong monsters, use them to win the tournament, and Janice will join your party.