This Seeking Tech guide will show you how to unlock the Leader of the Season trophy in Jeanne d’Arc. To get this trophy, you have to complete the first season of the Colosseum.
This location can be visited after the 15th stage of the main campaign. It is very likely that you will be under-leveled at that point however since the enemies will be as high as level 49.
Instead, you should wait until you are at least level 50 to take on the 10-round gauntlet. You should also make sure that you have the HP Plus and HP Recovery skills equipped. You should also have some attack and healing spells just in case.
The actual fights should be easy if you are at least level 50 with a good amount of health and healing.