Pro tip: the enemies NEVER goes in the angles so you can easily imagine what this means with non-shooting enemies... i suggest the south-west angle which is the deepest and the one from which you can see the whole arena!
why doesn't anyone understand... this game is NOT EASY sooner or later you wan't be able to do enough point to continue, the real purpose of the game is to score tha maximum point you can in the maximum level you can reach, then submit. I'ts not so hard to realize! -.-