Use this short and simple guide to trigger and complete Asuna Hollow event.

Asuna Hollow Event
There is a garden near Serpantis’s Sea of Trees
Go to the Forest of Elves alone through the Bewildered Forest
Then head back to Agil’s shop for the event
Party with Asuna and go back to the garden again and receive key item
After you complete the first hollow area, at noon time go to Agil’s shop to trigger
Bathhouse event with Asuna
An unimaginable gate was found near Ginriogia Void
Go to Ginriogia Stairway Remain F1 and go to southwest exit. You will find a door and go back to Aincrad and tell Asuna about it.
After that go back to that door for the event
Go to Ginriogia Void Teleport Board. Then use the teleporter and you will back in Ginriogia Stairway Remain F1
Just walk staight from the teleporter or go to the southwest exit.
Then you will find the door for an event. After that you will receive Resonance Ring
Asuna Hollow Event End

Source: http://www.gamefaqs.com/boards/734608-sword-art-online-hollow-fragment/69647786 by Sunsetdeska