Here's the sequence:

1) Free Penelo to get through regular Lhusu mines
2) Agree to do the Antlion hunt. On the Lhusu Mines you will find a kid who will give you the key to Area 9
3) Kill Antlion. The kid's brother will mention he dropped another key
4) Agree to do Diabolos hunt. Diabolos is just around the save crystal deep inside the mine
5) Kill Diabolos
6) Agree to do the Ancient Man of Mystery hunt. Go to the place you killed the Antlion and you will meet him. He escapes deeper into the mine
7) Go to the Phon Coast and find the "fallen Bhujerban" near a big boulder. a shiny thing on the floor near him will be the key to site 11 of Lhusu mines, where you can finally complete the Ancient Man of Mystery hunt.