Remember you also have Orientation Rolls available (General Perception usually but RMSS would have its various combat perceptions)
We tend to keep it pretty simple.
When it is your turn, do what you want, the lower initiative person hopes to survive with a parry and can then do what it wants. In the case of the dragon with its multiple attacks; if it wants to lay them all on one PC then that character's parry applies to all attacks. The dragon would be wiser to spread its attacks around to keep multiple opponents parrying. This is a feature of RM
1 opponent: some parry and shield
2 opponents: shield on one, parry on the other
3 opponents: shield on one, parry on the other, raw DB on the last... then hope you have a good healer.