Pretty straightforward by now... shoot them and don't get shot...
Wave 52
Stay in the safe zones and blast away.
Wave 53
Same in this wave.
Wave 54
Watch the bombs from the previous wave. These ladders shoot between
one and five long rectangular bombs that move at different speeds, and
faster than the other bombs, so watch for them.
Wave 55
The spiny ships now shoot blue drips that come down slightly faster
than the usual bombs. Be careful with these ones as you can easily misjudge
the speed and get trapped.