For those having issues defeating boss on Legendary, I was able to beat in a few minutes with a Beam Rifle.
Kill the enemies until the boss appears. To get a Beam Rifle, backtrack to the non-damaged building on the right of where you entered the final battle area and joined up with the Arbiter (when facing the direction of boss spawn). *Stay out of view of boss so he can't one shot you with his laser beam/purple orb attack.* Use blue lift elevator to 2nd floor and run back in direction of boss along walkway to the end and Beam Rifle is leaning on a box.
Run forward to the walkway of the damaged, tilted building to get on the right flank of boss. Hide and let the boss go after the Arbiter/spartan team so you can shoot the purple orb weak spot on its back with Beam Rifle (an Incineration Cannon/Fuel Rod is up there as well that might work, but not as accurate). I was able to kill the boss with just that and destroying the Watchers.
Don't worry about killing the other enemies (except Watchers) just stay out of their view and so they don't attack you. If the Arbiter dies, you will loose your distraction/bullet sponge and enemies will focus on you so might need to load last checkpoint at that point.