Rakshashas do have a glaring vulnerability which is definitely factored into their CR. It's neither intuitive nor obvious that... a specific thing kills them (spoiler avoided). They are also not combat-monsters. Most likely a group will know of the presence of a tiger fiend before actually meeting it, or they will be unable to affect it (with magic or weapons) and flee. If they were really on-task, they could cast commune at this point, and I'm sure they would be able to find out that they're up against a Rakshasha, and what its specific weakness is.
And for the rules on magic immunity, it is stated specifically that it counts as unbeatable spell resistance. In the example of claws, as long as they don't offer SR to resist being clawed, they work. Ditto for strange targeted spells like Maze.
-nameless