Time Leap is an extra campaign that unlocks in the post-game.
Enemy levels seem to be dependant on Fuka's level. I've tested on two files, one with Fuka at level 120, another at 1600. Desco was over 5000 in each file. Enemies started at level 90 in the first file and 1120 in the second. I would recommend avoiding powerleveling Fuka until after this and the Fuka and Desco Show are complete.
There are 16 missions, and the only story characters you have access to are Fuka and Desco, so you will likely need some strong generics or post-game characters to fill out the team. If you've been powerleveling in Mt. Ordeal, you should have some strong monsters, primarily a Cu Sith, Nekomata, and Deathsaber, and whatever monsters are being fused into them.
As for the stages themselves, there's nothing terribly difficult about them. You will get three additional story characters, which will only be useful if the enemies are low level. I will put them and unlock conditions in post-game in a spoiler if you would prefer to be surprised.