I'd say 7 powerstrike is enough, so 21/18 would have that, enough athletics and 7 IF on lvl 31.
I have a 9 PS 3 ath alt (lvl 27), hits hard, but only 3 ath makes it very slow. I even compared this alt to a 7 PS 24/12 STF alt just because I was interested on how big the 7 PS -> 9 PS difference is, found out that it takes me about the same number of hits to kill a guy, it was more related to speed bonuses and stuff I guess. Therefore I believe 7 PS would be enough and you also get enough ath to move around.