It's like most of the rest of the content in the main game, just that for me it was about 5-15 levels above where I was at when I finished the game. They do add some cheeky enemy combinations that make it more difficult than usual, but it's nothing that couldn't be overcome with level grinding. Problem is that the amount of EXP needed to go up from level 40-50 is (I think) more than it takes to go from 1-40, so I think it'd be a LONG grind to max level. Then finding the highest level weapons, or 5* weapons you can bump up to be actually effective. And grinding orb thingies to craft the best gadget upgrades. A lot of this looks like it could be done with replaying the levels on higher difficulty, but it looked like it'd essentially require playing every level again twice.
It's cool that it's there for people who love the grind, I guess it gives them something to keep working towards after the credits roll.