Generally, get a meta team ninjas (meta ninjas usually works well for long time, such as next fuku with 180 fragments of Kisame in it)

once you get a "dream team" you focus solely on power.
For F2P player, diversity and high amount of ninjas is luxury, (and be honest, you dont use more than few ninjas most of the time).

just keep grinding and freeze lvl, or you will get eaten by a us... I say us while I get eaten by whales, while Im at lvl 98 with 616k atm.
there is always a bigger fish :D

now without fun, get as much power and initiative on P1 as you can, get every free power up item in events and forget about plot instances or any source of unnecessary XP income (such as daily rewards, wanted missions and such stuff) ... its not worthy

That means, they dont whale too much.. Most ppl play either completely F2P or by buying monthly cards only.