I agree with some of what's been stated above....ninja are generally a waste of coupons (...unless you're a whale....then it becomes all about collecting and prestige).

The thing is, with the lengthy time it takes to save for a ninja, the more likely it'll become less useful (even useless) by the time you buy it. After a few months, better ninja are available, counters are developed...and in some cases, the one you want is nerfed.

Further, events are a slippery slope...it's easy to go to them for ninja frags, but if you're not careful, you can end up wasting coupons on low-yield gambles, or panic buys.

On the other side of the coin is battle power....it's not trendy or fun...or even interesting, but it accvmulates with time and diligence and works well with any lineup. That's what I'd focus on. As far as what to buy? Generally I recommend players focus on low-mid level items in every group (runes, mood, magatamas, etc). That's where it's most cost-effective. But I would check 2 places to help pinpoint where you should focus:

First, the game helps you. While you're logged in, in the upper left corner by your name is a red box labeled "Power Up" ...if you click it, you'll see what your weak areas are.

Secondly, in the lower right corner, click "Shop" and go to the "Purchase Limit" tab. Here you can get packs for varying power boosting items that are currently on sale. I like these sales better than most events because there's no risk of loss--you know what you're gonna get. They're essentially the same as what you'd find elsewhere in the Shop--just cheaper.

Case in point: I have 2 accounts, One is PTW that focuses entirely on collecting new ninja, the other is F2P and focuses entirely on battle power.

The PTW account is a lot more fun, because it has cooler ninja and more options for lineups. However, at the end of the day, the F2P account has an easier time playing, because it's a lot stronger for its level.
(why 2 accounts? I think of budgeting as being comparable to dieting. Strictly eating only foul-tasting health food isn't sustainable. Statistically most dieters cave in and binge on treats--whereas diets that allow for an occasional small indulgence tend to be successful. While I'm very active on my PTW account, I recharge very little--it's how I treat myself and keep my F2P account on track)

Anyways, I hope this helps :)