There's a few different things you could do.

1-Go for Kage, get a rare, then build around said rare until you get Roshi or better ninja

2-Go for Roshi, who's an absolute powerhouse and often used with Sailor Sakura and Water Main.

3-Go for GNW Treasure, which mainly has ninja that are blitz units...It's up to you

I'm sure others can give better info though