Apollon and I have been training together on two new Zeroes. We both played within a month of Zero's release in December 2013, yet the grind is still excessively tedious. The number of times I've entered higher-level dungeons such as Ludibrium dungeons as well as Road of Oblivion on my main Zero character peaks at over 130. That sole number is proof that I have endured the grind. Each dungeon takes around 2-5 minutes tops. Imagine doing that over 130 times consecutively. The reason why no one plays Zero for the content anymore is mostly because of this reason. Players can barely survive the mindless labor without quest 2x or other EXP boosters. I, myself, even quit Zero for a while during Temple of Time, but I still came back to make it to 180.
My suggestion is to raise the Zero level cap for leaving Mirror World either back to 180, or make it so you need to complete the level 168 storyline quest as well as to raise the EXP gained in Mirror World. Mirror World was meant to be Zero-exclusive content, and many players are missing out on the quests, content, or storyline that is present there due to the easy cop-out as well as the repetitive grind that easily shuts people off from Zero. The reason why Mirror World should be mandatory is because Zero has a skill that increases permanent damage (by 20% at max level) accordingly to the amount of Goddess Teardrops you obtain through the storyline.
Also, make each Mirror World dungeon a Party Play area to encourage players to work together in Mirror World. This is optional, but the suggestions discussed above is definitely something that Zero needs. Thank you for reading, and please consider my feedback with Zero as it is now.