Ranks doesn't matter one bit, just your ninja pool (and your level, up to 55). A lv 55 ptw with some rare ninjas at student level can easily * a lv 85 at 3 star six path if the latter doesn't have rare ninjas of his own(thou somewhat unlikely at that point)
At the same time, if a lv 70s only do the arena daily quest and missions, he might be of lower rank than someone who is 60 but actively climb arena but is stronger due to better collection of ninja.

The fact that people have to do the annoying arena missions also mean that many will be running around with sub-optimal teams, that can be taken advantage of if you wish to climb.

55 is what you need to be, both because main char summon/skill and also your collection toward the standard ninja sets. Generally speaking getting all ninja to 3 star to access their upgraded versions, where applicable, is all you need on that aspect.