Ahh.. I managed to survive on "Medium" again -- 18 lives left this time. The two final flying whatevertheyares were the only enemies to make it through.

Nothing but ordinary pellet towers, and the center filled with Typhoon squirters.
One improvement in my play is to beef up only the centermost pieces to the max before bothering with any other upgrades. One well-placed heavy is worth a zillion lower-lifes.

I guess I'll have to learn the other types of pieces to figure out how to stop those two final fliers.