This is how i do it, not sure if best way. Upgrade 1 item at a time as far as u can to make use of the wishing credits. I stopped at 9 though cos goin from 9 to 10 is pretty hard even with the wishing credit help.

If starting from scratch then i upgraded all the equip of my 1st position ninja and 2nd position ninja to +6. Then next event i took my 1st position to +8 and some to +9 to use my advanced runes whilst taking my 3rd and 4th ninjas to +6.
The reason i keep at +6 is so u c*e the next event to give u wishing credits for the +7 to +8 jump to help u save on advanced runes which r the most expensive