TL:DR: Attack from below if possible, shield, or use projectiles/disjoints to get to Mythra's hurtbox.
Before we get into counterplay, take a close look at the hitboxes on this move, courtesy of ultimateframedata.com (click the image for a higher quality gif):
There are a few things to notice here...
- Photon Edge doesn't have any hitboxes below Mythra (despite the flashy sword animations)
- Photon Edge has a LOT of end-lag--18 frames, or almost a third of a second.
- This animation looks cool as all heck
- Look closely at the above gif, the gray body in the middle of the screen moving from left to right is Mythra's hurtbox. That's what she'll use for hit-detection on any moves you throw out, NOT the animations of her teleporting.
All of this ties together to give you three main options depending on your character and the situation.
Attack from below if possible
If the Mythra starts this move in the air, you should be able to very easily get underneath them. The move from start to finish lasts 100 frames (1.66 sec). Since you don't need to worry about the slashes from underneath, you can use all that time to position yourself and punish accordingly.
Shield is your friend
A combination of very little shield pressure and a LOT of endlag means that shield is the perfect counter here. Not only does the move barely reduce shield, the last hit only has 3 frames of shield stun while Mythra will be in 18 frames of endlag. This should give you ample time to punish with something as strong and slow as a forward smash.
If you notice the Mythra is using this move a lot when they begin to approach, just hold shield when you see them coming.
Punish the hurtbox with projectiles/disjoints
As stated above, Mythra's hurtbox moves slowly forward in line with where she began the move. The individual slashes also will not clank with any other moves. This means that you can easily throw out a projectile to knock her out of Photon Edge. Or, if you have the timing down and a disjointed move (like a sword attack), you can swing through Photon Edge to hit her hurtbox.
Here's a slowed down gif of the man with the largest sword in the game doing just that: