Playing through on legendary dificulty. I swear those promethian knights can take 4 beam rifle shots to the face before dying. What weapons are good aginst them? Or are they just giant damage sponges with no quick way to kill them?
Ok so the promethians are easy to kill with the promethian rocket launcher thing. One hit kills them. Also the light rifle takes about a clip and a half if you stay zoomed in. The scatter shot is also decently effective. Any other ideas or input out there would be appericiated.
nothing because all you had to do is pump a certain amount of bullets into them. stupid enemy they are
One of the methods I have found to be useful is deploying the auto-turret as a decoy, then using either a plasma weapon or suppression weapon to bring down their shields and finally pop them in the dome with either the Bolt Shot, Battle Rifle, DMR, or Light Rifle.
Needlers work pretty well on the Knights
Oh yeah forgot to mention that the hologram seems to distract them pretty well. Set up some nice assisinations with that AA
2 Shots with the Scatter Shot does the trick. Anything else takes time. 
Overcharged plasma pistol
PP overcharge, or unload a clip and half of Suppressor into them, as sooner or later they will stop to throw a nade at you. Just keep moving and take to cover.
I causally Just get in there face with a over charged bolt shot kills them in one hit if all of them hit it , Its insane.
The Suppressor does a great job against Knights, the downside is the shorter range, but if you can throw a sentry turret out, or use hologram, that will distract them long enough to close the gap and melt their face.