The final fight against the level 100 boss has two parts to it:
- Kirito and his usual group of friends go up against it alone.
- Asuna, and everyone else from ALO and GGO join in.
In the 1st half of the battle, Kirito and friends basically get annihilated. In this first part, they observe that the boss will heal itself with a tree and drop of water.
In the 2nd half of the battle where everyone is in, the boss attempts to heal itself again. But Asuna tells everyone to block the drop of water to stop the healing.
Asuna wasn't there yet when the boss first healed itself. She wasn't watching either as she was talking to Yuuna and not in full dive.
How did she know about the move to tell everyone to block it? Is this a plot hole? Or is there an explanation to it?