This took me ages to find a suitable spot and was my last achievement, so hopefully I save you some time.
I tried this twice on normal and it didn't pop, so the difficulty on this video is hard. There may be enough enemies on normal and my timing was just bad. If you can get this to pop on normal, just post below so others know, but with max upgrades, hard isn't that difficult.
Level: 3-4 Terror in the Valley
Just wait until the end of the level where a wave of ships fly in from the east. Use the Sky Bomb II skill
when you think there's 35 or more ships on screen.