I'm not a main RP, but as far as I know... from 150+ the diamond arrows and AOE damage is always the best option.
So, if I was an RP I would use assassin stars in places where a shield is required, from level 80 until lvl 149... (although I cannot think of a single place where it would be superior than life leech/mana leech/critical in a bow)
For medusas I guess I'd try to deal the most damage, since they take a lot of damage for physical. So, I'd not go for assassin stars...
Try a bow with life leech imbued, crit and mana, if you are having problems with sustain. It may help more than the assassin star + shield.