Well i know of people that got every piece of darklight within a month, every single relic weapon.. bassically people that never leave the game other than to sleep for 2 hours. I would consider these people "hardcore".

Also- people that beat the hardest content within a day, but more often than not they are the same people described above from what i have noticed.

I wouldnt call someone hardcore because they completed any specfific content- they are simply playing the game.