Medica II is only good if your party is below 60% HP, Medica II can take precedence over Medica I in certain cases just because of the increased range it has. As far as Regen + Medica II before a pull, that should never happen IMO. Your tank should have a Stoneskin up making them useless for the first couple hits.

Single target boss opener: Aero III(while tank runs in) --> Aero II --> Aero I --> Assize(while running to boss) --> Fluid Aura --> Presence of Mind --> Stone III Spam --> Refresh Aero III and Aero II as needed. Aero I only when you have to move or weaving in Fluid Aura or Assize. Another time to use Aero I is right before switching out of Cleric Stance. I won't cast Regen on the tank until quite a bit into the fight. Once the tank hits 40% or 50% I'll just Tetra them back to full HP and do an Asylum or Regen after that, saving you MP and a global. You normally will have Tetra up again later when you need it anyways.