889 kills is awesome Jacobin..I do not tend to use the guns..Might now though, lol
Best for me is just a little over 260 kills with one unit of Naginata monks..achieved by running through the flanks of ashigaru and bow samurai..and then some easy pickings on routing enemy but nothing near that number..Nice!

Maltz..yes the AI seems reluctant to assault, My first couple of campaigns were on normal difficulty and the AI did assault then but only when they had at least 3or 4 times the numbers I did. (of course you would still win as the silly buggers fall of the walls at an alarming rate..)
However I have not had many assaults against me at all in legendary campaigns. I am not sure if this is because I am now better prepared for province defence or if in legendary they would rather burn crops and just wait for you to come to them...
On the rare occasion I have had one of my castles assaulted it has always been when I have had the sense of being under equipped with bow...I am wondering if the unit type is maybe a trigger then?