I hope this isn't really obvious and I just can't see it but I'm having trouble getting the particle system name. Actions like Get Name or String Switch ask for game object references or variables and I'm not sure how to pass them the particle system that's triggered the by On Particle Collision.
I've based my FMS on the one in the demo scene from the Shuriken Addon page linked in the first post. The FSM is applied to a child of my enemy game object (the enemy game object has a 2D collider already attached, as the game is mostly 2D.) It's laid out as in the attached image. This is all probably not relevant to finding a solution but I thought I'd share it just in case.