The blessed crossbow bolt-thing is kinda boring. I'm thinking more of "The poorest man in the land must give the rakshasa a kiss on the forehead, and if the rakshasa is killed by sinking a blessed dagger into its forehead no more than a day after, the fiend is permanently destroyed"
I like this sort of thing as long as the Rakshasa doesn't know about it, or failing that doesn't see that such a prophecy can be fulfilled in an unexpected way (the slaying of the Lord of the Nazgul - unkillable by any man - by a halfling and a woman is a good example of what I mean by this).
I would suggest that the Rakshasa should be unique and that there be a strong backstory to his character. You really should know who he was, and how and why he chose the path he did. How he will be destroyed should then be linked to that backstory. We're trying to tell
Bram's Stoker's Dracula, instead of a generic vampire tale, if you understand my meaning. Otherwise it doesn't matter whether you choose a crossbow bolt or a kiss on the cheek - it's may as well just be garlic, wooden stakes and sunlight for all the meaning it will have.
Secondly, how does the party learn of the necessary method of execution? Is there some prophecy? Have they found the diary of the Rakshasa's original lover? Or did they read of it in a legend in some dusty tome? If so, there may well be ambiguities there that they may not realise. Perhaps a word in the text had two meanings back when it was written that only a high 'languages' roll would reveal. Using the example above, perhaps 'kiss' only means to touch, as in the kiss of a cue ball and billiard ball. A kiss with the lips will certainly work, but is difficult to arrange, and might be shown to be unnecessary if the players take the time to research carefully! Certainly, a simple touch would be less likely to arouse the Rakshasa's suspicion (if it is indeed aware of the 'prophecy')
In any case, it would be really interesting to see how the group goes about their task. Again, using your example, what if they decide 'poor' means poor in spirit? What follows from there? Or, if poor literally means impecunious, how they go about finding such a man? Does he cease to be poor if the party pays him to carry out the deed - and if they don't pay him, how do they convince him to do the kissing at all? There are lots of interesting problems for them to work out. You might leave it all open by not determining the outcome in advance and just giving the party room to work out a solution and following their lead.
Here one way to set it up, based on your idea:
Keep the blessed crossbow bolt element as a link back to the original concept and to give the players a sense that you are not just making it all up! However, add a layer of complexity by requiring the bolt to be blessed by someone in particular. i.e.,
"An arrow, blessed by the most lovelorn woman in the land, shot into the heart of the Rakshasa, before dawn on the day after he is kissed by the poorest man in the land, will kill him and prevent his return. Note well, that the kiss and the blessing may not be forced, or the prophecy will fail"
Tying the themes of Love, Richness and Poorness (and even Reincarnation) into the backstory of the Rakshasa gives you a Prince who loves a Princess that spurns him for the love of a pauper. The Prince kills the Pauper (shooting him through the heart with an arrow) and then kills the Princess (who learned of his deed) by cutting her up and feeding her to his tigers. He then tries to kill himself, but is cursed to live on (unable to climb the kharmic ladder) as some sort of tiger spirit - or Rakshasa.
To tie things up really sweetly at the end of the game, the most lovelorn woman the players need to find also happens to be rich. During the game, she meets and falls in love with the poorest man, fixing both their problems and ending their curse - as of course they are the reincarnated princess and pauper.
