Use this short and easy guide to trigger and complete Leafa’s hollow events.
Leafa Hollow Event
Bring Leafa to Bay Area at Graysreev
Go to southwest area of Graysreev Beach and you will find pink ? Icon.
Then some enemy will spawn and you have to kill it
This complete the first part of the Hollow Event
The second part is located at Outer World Area at Arlebast => Laughing Ghost Meeting Place
A stone sword was found near Arlebast’s Outer World Area
Alone teleport to Perplexed Alien forest and go to northwest exit to enter Laughing Ghost Meeting Place
Then go to the right side of the map until you reach a big area and there will be pink ? In the middle of that area
After that go back to Aincrad and go to Agil’s shop to tell Leafa about it
Then party with Leafa and go back to Laughing Ghost Meeting Place of that area where you found the pink ?
After the event teleport to Graysreev Beach and go to the middle of southwest area to find the pink ?
Be ready to fight two lv130 statue boss. After that Leafa has learned a new skill
Leafa Hollow Event End
Source: http://www.gamefaqs.com/boards/734608-sword-art-online-hollow-fragment/69647786 by Sunsetdeska