Have your party consist of Hero, Martina, Camyu, and Silbia (no idea what the correct romanisation of their names are, sorry).
In the Hero trial, one of the outside areas (I think it is the 3rd floor, you go through a portal or something) there are these flame guys. They have weak AOE attacks that they use on your party. Fight them, destroy the non flame dudes if they show up, then just keep guarding and healing.
It shouldn't take too long to get your characters into the zone state. Once Camyu, Hero, and Martina are in the zone, use super roulette. It will increase the XP you get greatly.
After you use super roulette, switch Silbia out once he is in trance (I recommend for Rou to get him in trance too), and then keep guarding again until Martina is in a trance again. The hero has a move that gets him into trance immediately assuming you have got it on the skill tree. So once Martina is in trance, bring Silbia back in, and use Spectacle Show which requires the Hero, Martina, and Silbia. This will result in the enemies becoming either metal slimes, or shitty robots. If you get the slimes, you want Camyu and Martina to use their guaranteed critical attacks, and if you can get Rou back in and he is in trance, use the lullaby which puts the slimes to sleep. Don't bother attacking the slimes with anything other than crits as it is a waste of time. After you kill the slimes you will get mad XP.
example of mad XP
If you got the robots instead just try again.
Once you hit level 70, the metal slimes will show up with a metal slime king. Super Roulette guarantees you an item drop, and the item that the metal slime king drops is an item that puts your party members into the Zone, making the whole process faster.
It's not exactly an exciting grinding process, but it is pretty fast. I went from level 60~ to 99 in like less than 4 hours.