In Sword Art Online Ordinal Scale, how does Asuna know about the boss' healing ability? 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?
If Asuna is well-learned (or more likely just because she played ALfheim Online, whose lore fiddles with Norse mythology), she may have known that in Norse mythology, the dew that falls from the tree Yggdrasil carries the power of life. In that case, she must have connected the dots instantly after seeing the tree and the dropping dew, and/or perhaps also after hearing the healing SFX that started playing at the same time.

It is interesting to note that Yggdrasil in ALfheim Online shelters the capital city of the game, and that Asuna was also a hostage at the top of Yggrasil for a whole cour.

From Prose Edda/Gylfaginning, 16:


  An ash I know
  
  Hight Ygdrasil;
  
  A high, holy tree
  
  With white clay sprinkled.
  
  Thence comes the dews
  
  That fall in the dales.
  
  Green forever it stands
  
  Over Urd's fountain.
  
  The dew which falls on the earth from this tree men call honey-fall, and it is the food of bees.


From Prose Edda/Gylfaginning, 58:


  In a place called Hodmimer's-hold are concealed two persons during Surt's fire, called Lif and Lifthraser. They feed on the morning dew. From these so numerous a race is descended that they fill the whole world with people, as is here said:
  
  Lif and Lifthraser
     Will lie hid
  
  In Hodmimer's-holt.
  
  The morning dew
  
  They have for food.
  
  From them are the races descended.