A realistic lens flare can be achieved within the Reach profile by being "physically correct" and performing a check of each frame to confirm that the source of light is, at least partially, visible from the camera's point of view. (Tests involving whether something is in line of sight from something else are commonly referred to as occlusion tests.)
The bad news is, that without the benefit of hardware accelerated occlusion testing like the HiDef profile's
OcclusionQuery class can offer, this test can be beyond the processing resources available to a game (especially one running on either mobile or older hardware).
The good news is that we do have a relatively cheap alternative that may just provide ...