My current build is 21/15 with 7 ps, 7 if, 5 shield, 5 ath, 5 wm and 5 riding (and i'm lvl 32).
Of course, you don't need the 5 riding for siege and apparently you don't want wm (even if, in my opinion, it's very important, realy feel a difference between 120 and 150 wpf). So you could go for a 21/18 build or even a 21/21 build.
Actually, it depends of the weapon you want to use. I currently have a mw warhammer, i often one shot poeple and i need a maximum of 4 hit to kill str builds or full plate guys. It would be an awesome weapon if i had more ath to compensate the length, so if you want to use it, you should get more ath than me, at least 6 or even 7.
As far as ps is concerned, i think 7 is enough, i also had an elite scimitar (with the same build) and i have now a ncs, and i often 2 shot poeple and never (but for str builds and full plate) need like 5 or 6 hits to kill someone.
Ath is also very important (imo) when you are alone against 3/4/5 ennemies, so you can backpeddal until you find the right moment to attack the one who seems the most unprepared (don't know if it's correct english here :s) and then deal with the rest.