I did it as Sakata said, got the lv 10 ones first, then lv 11 then lv 12. (And I was actually lucky enough to get a freaking lv 12 agi stone .)
I'd think 4 Lv 10 stones will improve you more than one lv 12 stone. (assuming that u have use for all the stones u get)
(Side note: my lv 11 stones were BOTH dodge stones. But that was ok for me, cuz I was going for ex-skill starrk)