Scene #17  Death has many faces

I believe you can see Arya enter the woods just before Theon's men are attacked.

It occurs when Theon hears something and sees shadows in the wood.   He says “Get Ready”.  I think Arya is the first shadow seen by Theon.  She is being followed by one or more undead.  One of whom she will use to disguise herself, so that she can get into position without drawing attention.

When the undead start to come into the scene seconds later they come through the gate and directly attack Theon and his men, as they usually do.  No strategy just aggressive straightforward undead attack. That’s why the first few didn’t attack.  … they were chasing Arya.

When this attack starts there still aren't many undead around.  This occurs during the great lull in the battle where the living have cleared out most of the undead. And before the Night King raised the second wave of undead.   It’s during this phase that Arya is getting ready for the most important assassination attempt of her life.

(So, she was in place earlier then I thought.)

Also in this scene you can see the iron bucket she will uses to increase both her jump height and distance.  Theon was using it to hold his arrows.  But it'll be empty when she uses it for her jump.

So, I believe, she got to the forest when there weren't many undead around and she waited.  She may have prepared a disguise, While I don’t think she needed one, I think she did,  because it would have made it easier to just be standing there with the rest of the regular undead, and it would have allowed her to get closer to the Night King. That and the title of the scene #17 Death has many faces would kind of make sense.

How did Arya's attack go down?

Facing the tree, her attack came from off center and to the right of where the Night King is standing over Bran, maybe 10 yards from where Arya is.

She ran from her position slightly off the right shoulder of the NK, she jumped on the edge of the iron bucket which added an extra 24"-30" of height to her jump.  The average girl her age, in good shape, can jump 20" vertically.

I think that would have given her a maximum jump height from her feet to the ground of 44" to 50" At the peak inflection point of her trajectory, that put her head somewhere between 2.75'-3.25' above the head of the Night King. ...well represented in the shot we get to see.

https://thecrazywatchers.com/wp-content/uploads/2019/04/anigif_sub-buzz-6273-1556511276-1.gif

Yes, with her arms extended at the mini-max potential of that trajectory's inflection point she could have dunked a basketball. Oh, and per her usual practice she ripped off her mask just before she killed the NK.

I also think we'll probably get a reveal.  So maybe we'll get to see if I'm right or not.  Which would be nice.

If you go back and watch from scene #14 titled What do we say to the god of death everything I've written is based on evidence that can be seen in the episode.  It's just not in your face obvious.  There are also a lot of cut-scenes so you have to watch carefully.