If u want to level up easy and ur a low level guy like aorund the 20s the 10s or the 1s then train at sulfrous valley where u get the secend mirror it worked for me and now im kickin butt also
i say that u should train up levels before getting each mirror cuz the phantoms keep getting stronger be careful and train hard PS u should train in the room outside the room with the mirror and i mean right outside it cuz u will be training next to the heal and save thing so after each battle u can heal ur guys and save every once and a while

HOPE THIS HELPS!