I managed to get 52 as well. After trying for some time without luck I found myself a method. A number of deaths later and some practice at not panicing and having fast reactions and I finished the level to record 991. I think I had 368 gems but did take it a little too cautiously.
I managed 371 gems and a much faster run but died right near the exit.
Then it happened!! Just a freak of nature.... 379.... I thought it was a typo but no, 379. "Unfortunately" I didn't get it perfect and ended up with the aliens in a line (bottom right), but did get it right enough to get all but one gems and have the other aliens held back (to the left). I also had heaps of time (over 20 "spare"). Being very patient I hid in the gems to the right (fortunately there were 6 or 7, two levels deep) until I figured it was clear enough, then moved the aliens into position, grabbed the last couple and exited with 1014.
And so I cracked 240.... I just need ten more and we only need 4. Of course it's possible!