Hi! I may be able to help I was in a similar boat to you trying to figure this out. Little late, 3 days later. You may have figured it out by now but I hope this can offer guidance to anyone else who runs into this issue.
First off your level shouldn't matter to much so long as you can do enough damage, however you need to play this like it's Sekiro.
You will likely need a "Defense Technique ring" (the combination of Technique charge parry ring and Quick Illusion rings) :
Technique Charge Parry would be a minimum to S rank the fight if you want to keep damage up while not having to play super safe around all her animations. However, just to clear the fight I say you need Quick Illusion. When I first tried this fight I did not use a quick illusion (short mirage) ring (and didn't have a defense tech ring yet only a tech parry ring), and ran right into the issue you are having now; Where when she does the three dashes and the 3rd one would always hit at the end of the mirage/illusion animation. I'm unsure if there are gods out there who can do this with normal mirage/illsuion instead of short mirage but I highly suggest you get "short mirage(Quick illusion)" and "tech parry" combined ring if you don't already.
Once you got the ring you should practice evade canceling with it. Where you jump and evade in a direction right before you land, then jump and evade again. This isn't 100% needed to win this fight, but it will keep you ready to react to whatever comes next and personally I have found it to be a sublime tool and kept me safe until phantom came out.
Second tip: Lock on. This will help you track her movements. At the start of the fight you can lock on before she drops down, and personally I tech parry her fire burst opener with Shifta.
Now that we got that out of the way Techniques:
-Deband Don't bother. Even if you can keep it up, you're best to treat ever hit as a 1 hit KO.
-Nabarta can save your life, and has a weird interaction with her dash attack where if you block the first the second attack wont hit. But the biggest boon is Nabarta can be the thing you pop if you have a ton of PP or are using the restore, and the damage it does is pretty nice.
-If you're good with using Ilbarta to snipe the last hit I'd recommend that, but honestly if you want to keep it simple and not manage that during such an intense fight: I suggest you use Lighting, Dark, or Light for one phase, and switch element on phase 2. First time I S-ranked I used Dark on the first phase (Megid, Samegid, Gimegid if I knew she was going to be still.) and Lightning(Gizonde) on the second. Remember that she gains a massive damage resistance to the first element you used on her on phase 2.
Also note: DO NOT HIT HER WITH YOUR ROD OR TALIS. There seems to be some interaction where on phase 2 she resists all your elements and anything coming form that weapon type if you do. I didn't test this further but if you rely on that to regain PP or you have Photon bullet- DON'T.
-Combined Techs I only had the major ones when doing this so I cannot say for the simple combinations, I'd suspect Rebarantsia can give you some decent repositioning, and maybe the evasion on Refomelgia can save you, but again I never had access to them when I did it. as for the big hitters, honestly unless you got something really good lined up Fomelgion and Barantsion will likely get you killed. Zandion is the only one I've had any consistent use with and most of the time she's too jumpy for it to be all that effective; and there have been several times where she'd bomb me at the end of the animation, or it would leave me in one of those red circles on phase 2.
Actually defeating Phaleg: (copy pasta from another post I made a while back when ppl were asking for help)
Break down of her moves:
"I can run circles around you!" - Ranged stun with a lengthy charge time, You can see the energy gathering closer giving the obvious tell. You only need to dodge when she fires meaning if you feel you can get a few hits in go for it.
"You wont escape!" - Two energy blades in both hands as she runs at you for a multi hit kill combo, just get out of the way you can dodge through or past her if you need to. Follows up with "How do you like that!?"
"There!" - Does a pretty evident clock wise twirl before charging at you with a upper-cut kick, This should also be slow enough to dodge and also gives you an opportunity to get some damage in. Follows up with "How do you like that!?"
"How do you like that!?"- Jumps into the air and lands with a fire blast. Dodge or be out of range of the fire blast.
"I have you now!" - dashes forward twice, kicking forward sending out an energy wave, she maintains her trajectory if you avoid one blade you can likely avoid the other.
A lot of her moves come in pairs so watch out for it.
"Ha! x2-3" - (hardest move to deal with imo) This has no audio tell before she does it. She crouches down and dashes to your current location regardless of altitude shouting "Ha!, Ha!" You need to evade this and learn it's timing to do it consistently. Sometimes she will do Two Dashes and follow up with one of the other attacks. OR will do three dashes in a row. Either way they often (but not always) come in two sets of three.
be careful for the second set, you will often need to wait for it to finish, just because she did a move you're familiar with doesn't mean she's open; it's dangerous to get into an offensive mindset after the first set. She will often say "Are you done?" when she's vulnerable after the second set giving you a chance to get in some hits.
Pattern Examples: "Ha!, Ha!, You won't escape! (then) Ha!, Ha!, Ha!" or "Ha!, Ha!, I can run circles around you!, I have you now, (then) Ha!, Ha!, Ha!". Often finishing with "You done?"
Phase 2: Her follow ups seemingly come out a little faster, (pretty sure) she will do increased damage, and whatever weapon you used to damage her with (or tech element) she will have resistance to. You now also have stage hazards with red circle rings on the floor, and she's not afraid to double up on "I can run circles around you." -Warning-: She will break target lock-on just before going into her new pattern prepare to re-target.
"Do try not to die" - (her only new move in Phase 2) She will shoot a line of fire at you three times, each one following a short repositioning dash, just be patient and dodge it, wait for the third (and any possible follow ups) before DPSing her, She is usually vulnerable after this. Preferably dodge towards the side, if you try and evade backwards or forwards you may be caught in it's hurt box.
Extra tips: If you're struggling to get that S-Rank you can grab damage buffs from Alliance and drink on the bridge before each attempt. Don't bother buffing for defense you should play assuming every attack is going to kill you anyways.
This is how I beat her; Hope this helps.