if i were u i would fully finish level 6s across my ninjas and then break some in order to lvl 1 7 into an 8 on my m1, then get all lvl 6s again, rinse and repeat until you have all lvl 8s on ur m1. after that turn your lvl 6s into lvl 7s and then repeat step 1 breaking lvl 7s to turn 8s into 9s. thats what im doing currently, yes ur m1 needs high init, but it also needs high base dmg and crit/injury so u c*e edo hiruzen and insta kill ur opponent.

i would not work on getting lvl 8s directly as it takes a LONG time and during that time u can lvl other runes up so u gain small amounts of power while waiting to make ur m1 more op.