Not really a hard achievement, but it may take a few tries.
Save before entering the Sunken Cavern!
The section this is about is a side-scrolling section. Basically, just stay far to the left and spam Mermaid Bubble while moving up and down.
Be sure to handle bombs, but don't shoot them more than enough to destroy them.
Watch for Tinkerbats that run along the bottom, they can be a bit harder to see.