You must have a level 17 Palace or above and for level 1 complete ten daily quests; for level 2 complete fifteen daily quests; for level 3 complete 25 daily quests, level 4 complete 40 daily quests; level 5 complete 55 daily tasks.

There are also youtube videos coming out now with these numbers.

do 6 daily quests in a day  for level 1

This may be how you got it, however there are only 6 daily tasks, and I have never done 500 legendary tasks,so have never finished all 6,and I would be hard pressed to say I definitely did 5 of them ever,and a few days ago,much to my surprise I found that I had Sisyphus 1. 

You don't have to do ten/ fifteen/ twenty five/ forty/ fifty five all at once, you can accumulate them over a period of time.