Go for the Kage treasure as that has some pretty good water ninjas (Ao, Mabui, Mei, Darui, Chojuro) or you can go for the GNW Treasure and get Mangestsu, Fuguki, Edo Zabuza, and Edo Haku. Jin 1 Treasure was also pretty good for a water, I had a lot of success with a Yugito, Yagura, Roshi lineup that completely destroys Sailor Sakura teams (Poison tai from Yugito 20 combo chase and ignition from Roshi and Yugito Mystery). As far as event ninja go, you can't go wrong with Sailor Sakura.

As far as summons go, I found most of the time I was using Gamaken and Monkey King (Gamachu and Ninja Monkey being their purple equivalents). But honestly you should have all the purple and blue summons and a couple of golds by level 80.