Alright for all of you playing on Legendary here it is.
I just finished Cortana on Legendary singleplayer. The easiest way to do this is to use the numerous cloaking devises laying around and needlers/energy swords to fly through the level. I just complete the level in 45 min. It was actually I think a really short level...
Main thing, is keep on the move, if you stop, your done for. The best weapon for those Flood Snipers, what ever you want to call them, is the needler.
Just remember like any level, when playing on Legendary, don't try killing everything, just try to do what you can to make it from checkpoint to checkpoint!