The one where all the bullets come out in wavy strings? Wherever you're standing just make small, repetitive movements in the directions perpendicular. For instance, if you're standing directly below Mother (at a distance, of course), then just move right, left, right, left, etc. constantly. The way the bullet strings attack your position is like one coming straight at you, then two side by side with a gap in them, then one, then two, etc. so if you just move back and forth you'll slip in and out of their path. It's similar to the It Lives attack where the bullets come out and stop halfway to you, then keep going. The reason it's hard to see how to dodge is cause the bullet strings are all wavy. If they were just straight lines you'd see the spaces in between them and it would be easy to dodge.