I found the key was to head forward and to the side while firing nonstop. The way the shots loop around the tiny world allowed me to take down the portals and evil clones without having to face them head on.
Fun game! One issue, that you can see in the picture, is that the Jam flair covers part of the playing screen. (I wish they used a shorter title for their jams but that's another matter.)
I didn't realize for the longest time that there were seven waves total. At wave six I thought I was done for, but then I managed to make out the numbers in the upper right, which gave me the willpower to see things through to the end.