Gear Up a 5th Level Ninja

[Shellman]

Since the primary abilities for a Ninja are dex and wisdom

Definately buy a light weapon (i.e. rapier, shortsword)

Get the Weapon Finesse feat at 6th level = adds dex mod to BAB instead of str mod

Take improved critical at 9th level. (Rapier crit range is then 15-20)

Take Telling Blow at 12th level = applies SA damage on a confirmed critical

Gloves of dex +2 (applies to AC and BAB)
Pariapt of Wisdom +2 (applies to AC and Ki powers per day)

Remember the ki invisibility ability is for 1 whole round, not necessarily as the invisibility spell.

 

[brendan candries]

Telling Blow (PHBII) has Sneak Attack as prereq (but ninja's are treated as having SA for the purpose of feats/prestige classes) and can be taken anytime.

Improved Critical with a bab 8 prereq won't come sooner than level 11/12. By itself not a good feat for a finesse build imho. A keen weapon would net the benefit faster at a higher cost but saves a feat.

Straight ninja's don't have access to rapiers (or any melee martial weapon besides the short sword).

So a dip in a martial weapon class seems desirable if you want to follow this route. Swashbuckler would net Weapon Finesse automatically and has more skills than a fighter.

Conceptually I don't see a ninja and swashbuckler as a good mix though, but yeah.

Another feat that might help out here is Powered Critical (Weapon Focus; bab +4) granting a +4 bonus on threat confirm rolls.

This is a very random approach to deliver SA damage though. Either you try to deny your opponents their dex ac (with the options that the game provides) making all the investments described here rather moot. Or you hope you crit a lot, doing almost no damage in between.