I focused on ninjas myself. However after I got all my (or most) of my shop ninjas to 5 star I started going for power. After obtaining 113 ninjas I been saving coupons for power. Mostly for Lucky Board. Very rarely will I spend anywhere else unless I need like 1 Seal Scroll for a Draw 10.

Just be patient. Focus on your team you shown in the screen shot. Then starting getting power. I recommend buying Cave Key Gift Pack to start off with. It is so worth it.

Also don't settle for anything of blue quality or below on the initiative Purification. Go for Gold or purple at the least. Then pick up some Quick Runes from 8 gates shop. If you can do this things will get a lot easier.

Also you can beat your ninja exam, check out Will of D on youtube. Of course you can do it with Wind Main. You also may have to swap earth for 140.

You can however beat 180 at 53k power. I know I did. So don't ever say you can't do something. The only time you can't is when you don't try.