Rakshasas are your Intellectual Demonic Villains. They are masterminds, manipulaters, seducers. Your demonic vampire, if you will. Thus, they're going to be Prepared.
In response to the 'Common Knowledge of Crossbow Bolts', if Rakshasa were anything in this world, they would Pollute the knowledge. They would change it, fake other weaknesses. Stage fights, whre they were 'shot with a blessed bolt' and it didn't work. Use illusions to fake other weaknesses, and 'disappear' (Any good Major Image will certainly help that). If one's been killed with a Bolt (And sent home), then no doubt others will learn. For example, if they rule somewhere, have all the mirrors of the kingdom banished. Then when someone shows them a mirror, they can smile and comment on their looks.
Next, about the 'Insta-Kill' factor and their weakness, you know, having 'Insta-kill' or 'permenant death' is a possibility. I've heard somewhere (Is it OA?) where a ghost killed with a Jade weapon will not come back within 1d4 days. It suggests there's more material there, some mystical purpose, that does this. Something off the beaten path like brass, or oak wood, or having a symbol painted onto their forehead, whathaveyou.
And, about Rakshasa being insta-killed by a crossbow bolt, how about Each Rakshasa having a different weakness. Hit with a stone from a saint's burial site, their own reflection (Someone mentioned that the other day), Running water, Sunlight filtered through glass, etc.