Well first, screenshot what ninjas you have. I'm guessing you don't have a lot of great ninjas and free to play player. Since you're a water main, try and go for kage and pull for mei. While you do so, you can pull good commons for the meanwhile.

OK I was correct. Pull from Kage treasure as you can pull common such as Ao and Mabui and rares such as Mei, Chojuro, Killer Bee, and Darui. The ninjas I listed should help you at the moment.