You can do the "Quest" only once, of course, like any other quest.
You can do the "Battle" 3 times a day, you still get exp at the end, but probably less than the quest the guy at the mercenary post gives you.
You still get those shadow fragments at the end, that you can give to Brynn that gives you good exp (don't remember exactly, but it's around 45k i think).
If you want to farm exp, labyrinth isn't the best way, since it's 3 times a day and takes some time to get to level 41 even with hacks.
If you plan on hacking, you're better off doing a raid, 1HKO the boss, rinse, repeat. You will get as much exp as doing labyrinth in the same time, and you don't have a daily limit, except when tokens are back.