It would be best to kill off the henchmen before fighting Hikaru, unless your a completionist like me and are going for the Kami Seal, then just avoid like usual

Those light Orbs that float around will attach to you and detonate after a few seconds while draining your essence, so if one gets to close you'll need to sprint a few meters to shake it off which will get you spotted but after they are shaken offf you'll be able to go into the nearest shadow to get off prying eyes.

Hikaru will charge you if you are spotted so if this were to happen you'll need to teleport yourself to the nearest shadow in order to avoid the Light Beam

Ok, to start with you'll want to take Hikaru over towards the left side. While over there destroy the orb nearby then hop onto the ledge right above that area after leading him closer
FIXED: If you attempt to do this the other way around by backstabbing first it may glitch and he'll stop moving and you won't be able to get him under a ledge
You can now do either the backstab or aerial strike first

After you do an aerial stab or backstab , you'll be teleported to relative safety in a nearby shadow and Hikaru's backpack should show a Light beam shooting upwards or out the back depending on which location you struck frist

You'll want to go to one of the other orbs but the closest one is under the overhang nearby the first

After destroying that(or the other) orb you'll just have to do the other stab that you have not done yet, for example, If you did the aerial stab first you'll want to do the backstab or vice versa

After you stab him you'll see his back show 2 lights instead of one

It should show a cutscene where his backpack will explode and he'll be without protection

After that destroy the last orb and then just stab him one more time and then a nifty cutscene will appear and a animated battle will ensue

FIXED: I found this out after restarting the chapter a few times that when you fight him and do a normal backstab all is well and dandy, but then if he spots you and charges he may get glitched and stop moving and I mostly only see this during the second part after the first stab, and if you were to just backstab him the first time around you won't be able to land a aerial dive unless he's under something that you can climb onto...

I had helped a discussion thread where this problem had risen:
http://cs2bus.com/app/280160/discussions/0/350544117175710202/#c350544117175784616

If I helped anyone.... Your welcomes :3