This achievement can be very easily completed, and has a couple different ways of going about it.
Wait until night, as phosphor slimes will drift in the air for longer than other slime types. This can also apply to dervish slimes, and largos that are made with either of these (The feral Pink/Phosphor Largos in the cave towards Indigo Quarry comes to mind.)
You can also play god, and shoot them in the air yourself! Follow that bad boy up with their preferred food type. If you're feeling saucy, you can try to simultaneously complete 'Fireworks!' with this one.
Here's my video for every achievement if you'd prefer. Skip to 1:17 for Catch!