Best strategy in my opinion for Legendary and any difficulty is to take a
Sentinel Beam and an
Energy Sword before triggering the fight. You will want to go on/near a balcony as they provide infinite ammo crates for the Beam.
Just melt every enemy that faces you. don't forget to use your scanner and shield to help you, and once you finish a wave, take out your energy sword. Rush the Harbinger, take down her shield with the sword and then melee her with the Sentinel Beam until she flees. Rince and repeat for the 2 next waves.
For the 3rd and final wave, she will send a Brute Chieftain, with his trusty Gravity Hammer. Do your best to avoid him, kill every other enemy and keep him for last to avoid getting shot while you beam him. Good thing to avoid him rushing you with his Hammer is to use your Grappleshot to switch between upper and lower area.
Once you finish the wave, it's time for you to deliver the last blow to the Harbinger.
Congratulations Spartans, time to Finish the fight.