Power up your group skills more. They help a lot. In addition, refinements and ninja tools add on as well. It c*so be the star level of his characters too. The more stars a ninja has, the bigger boost they get to their stats.

Have you also updated your characters? You can do this in the "Train" section. Also moods help too give a big boost to a stat. For example: I'm a water main, so I changed my mood to boost ninjutsu and lower atk. I boosted the mood to level 6 (I think) and it gives -5% atk and +25% ninjutsu. This boosted my power by a lot.