In order to keep a relatively high AC, str 13 and a high dex/int is necessary. Since right now there's no race gives a bonus to both Int and Wis, the better races might be the ones that grant both Dex and Wis.
13 14 15 8 15 10 -> lv 4 13 14,18,8,18,10
Feat: Hide+Light Shield+Ritual Casting
AC: 10+2lv+4(+1hide)+1(light shield)+4Dex=21, the same with Dwarf Dru
On the other hand it seems like any attack to a spirit is an automatic hit, while the damage is reduced by 5 to the shaman itself, without any chance of defense...so ironically there might be no need for shaman to increase its defense at all...
I think I'll just suck the 4E D up.............