welll. the rangers crit is based on the ranged combat skill...
SO... we start our ranger as a mook Psionic, which gives us the ability to start out with about 10 more int and sen than any other race/class combo (including ranger)
we pump dex (nessasary to change to ranger at lv 1/2) and int and sen (spd is low unofrtounatly)
when we lv the first time we change to ranger by puting 3 into dex and sen and then pumping sen/int(until 95 because of incident in trynton) then dex/spd.
In order to get ranged crits we have to use bows or modern weapons, so i suggest using bow early (dex/str which you have plenty of) and modern late (spd/dex) which you will be pumping after lv 8 or so(spd will max at 28 or so
).
we wont be the fastest ranger around, but you should have an insane chance for ranged crits very early, and you will have powercast to let your ranger cast effectivly (so long as he/she practices spells) nearly a few lv's after you start learning them
the ninja/ranger class change doesnt work as crit skill only works on thrown (If ninja) and meele(for all)