Probably split them up evenly between Cave (if you have level 5 mag, you want to be at least at level 4 cave), purification (you want purple initiative on your best runes for PVP, plus purple on a few more), BA (you want to get level 4 BA during the BA rebate), and/or Medium Refine (you want to get more runes to level 6-7).

Though it won't raise your BP that much, initiative is incredibly important. higher initiative is worth 1-5k BP depending on the match tbh.