GMS v107 - I'm not saying it's aliens...

There was a repeatable quest in the alien's HQ where you could infinitely get 70k exp (if you keep going to the map).

The boss? Because I could only enter 5 times a day, it even gave me the notice. You gotta kill the driller boss 300 times anyways for a B rank nebula. Don't quite see how that is a exp exploit.

edit: Oh nevermind, I guess I rushed to complete too fast w/o caring. I already completed all NLC new quests.