I get that shaapa is almost surely "more difficult than most" to assassinate. Probably "very much more difficult than most". However, I'm not aware that any of his previous characters were
specifically targeted by skilled assassin players. If nobody was making a special effort to kill him, then he may not have had any skilled enemy assassins for most of his character's life. Obviously if there's nobody who knows what he's doing trying to assassinate you then you're much less likely to be assassinated.
Odds go up even further if the person playing the assassin has as his sole goal to kill shaapa. That is, he's not deterred by full loots and/or massive loss of CON from getting bashed down after failed attempts.