The difference of between squirmy finishes is based on what weapon the downed character is holding. There is a total of 30 squirmy finishes in the game. There are 3 for every weapon based on what body part you spray with the peep peep. I went about achieving this trophy by doing the following.
On the 8th episode of the gessen arc you face the Hanzo team Asuka holds dual pistols by default. So, grind her for the 3 dual pistols squirmy finishes. On the 9th episode of the same arc you face the Hebijo team. Miyabi holds a pistol, Ryoki the sniper, Ryona the shower gun, emu the shower gun, and Muraski the Gatling gun.
On the 8th level of the Hanzo arc you fight the gessen team, their weapons are: Yumi with the assault rifle, Murakumo with the grenade launcher, Yozakura with the shotgun, Shiki with spray gun, and Minori with the rocket launcher.