RS = Raging Strikes (cross-class skill from Archer)

There is no rotation, per se.

People do Bio 2 -> Miasma -> Bio -> Fester because it allows you to get in a second Fester before RS ends and maximizes your DPS. Sometimes you just do Bio 2 -> Miasma -> Thunder -> Bio -> Fester, for ease of use, especially when RS is not up.

Playing SMN is more about knowing what to use and when. Single Target (ST) fights are different from multi-mob fights where you employ AoE. What kind of AoE and how many DoT sets depend on the number of mobs, how long the fight will last, etc.

Your goal is to create a scenario where you get the most damage out of your DoTs as possible, and not cast them if the potency you net from them is outweighed by doing something more direct/immediate like Ruin or Tri-Disaster.

Sometimes your 'rotation' is dictated by the needs of the fight.. like you may need to lead with Shadow Flare (with Swiftcast is usually a good idea) to mitigate initial damage on the tank. May need to use Miasma 2 between DoT sets in order to mitigate healing. May be a good idea to multi-DoT with Thunder, many other times it is a waste of time and mana.

A lot of SMN spells (mainly DoTs) can be a complete waste of time and mana if cast in the wrong situation. It's going to take a lot more than a simple rotation to explain all of that.