There are 9 characters that must fully vermify:
Aya, Rico, Rucha, Edward, Eugene, Michelle, Komachi, Vic, Balbagan
Because Komachi is part of this list the earliest you can get this achievement is when you return to Kolton.
The area that I have observed most frequently gets Lunar Rain is just outside of Port Zala. Kill the two birds there and hide off to the side path that leads to Bihar Mines.
Take each character one at a time for this. You don't have to bother unequiping their weapons as there is a neat trick to this.
Once you have killed the two birds, sheath your sword and walk up the path to Bihar Mines so are out of sight behind the cliffs. Walk slowly and wait until your party member is directly behind you. Now turn around quickly. Your party member should have turned around and now be facing away from you in the direction you are looking. If not, try again until they are.
****Now put down your controller!****
Stand there in the Lunar Rain, and they will eventually turn black. Since they are facing away from you they don't know you are there and they won't find you because they don't move on their own. If you make the slightest motion, they find you and attack you (thus why I say put your controller down).
Once they turn clear (invisible), they have fully vermified. Run past them (which will scare them and the question mark will appear above them as they weren't aware you were behind them), run to Port Zala, rest at the inn (to get rid of the vermification effect) and switch party members. This takes about 45 minutes to do all 9, and is risk free if you use my trick.
[EDIT] It is possible for this to work with a full party (3 at a time)! Thus cutting your time required for this achievement to roughly 15 minutes! Just ensure that when you do the spin around portion of my trick that every character in your party is facing directly away from you. Wait until they all fully vermify and then run back to Port Zala.