Youve got all the advice you need, but ill just add that soloing an Assassin is fun!

Also, you might want to consider getting the Girdle of Hill Giant strength right after the Ring of Air Control...the bonus to damage will multiple nicely with BS
btw, Dundee said the Assassin gets better backstab later...this is true, but only if you have ToB, IIRC. In vanilla SoA, Assassins only get poison weapon and +1 hit/damage.(if you're gonna use a longsword, consider choosin elf as your race, 'cause that way you would get an additional +1 to hit/damage...)