I've finished the game solo with the Elf; what Morrigan said is pretty much the way to go. Move a lot, dodge like mad, and line up charged shots. I'm particularly fond of the charged air-to-ground shot, which can be done repeatedly by alternating it with air attacks (quickly interrupted into it or you'll fall down quick), double jumps and dodges. Ex. jump, charged shot, attack, charged shot, dodge, charged shot, attack, charged shot... You can stay airborne (which usually means out of trouble) for a lot of time while dealing heaps of damage.
Imps are kind of a pain with the elf indeed, as lining up shots isn't the easiest of tasks. The most important thing is to watch their shadows to see what plane are they in, and align yourself accordingly. Then you can either air shot horizontally (quite hard, requires you to be at the same height AND depth as them, but useful against groups of them clumped together), ground-to-air (relatively easy to hit them with if you're below them), or double jump and air-to-ground shot them (also relatively easy, although the ground shockwave typically will miss them). Once they're downed a charged air-to-ground shot usually kills them.
