I would say invest it in lucky board

Can you elaborate a little to support this statement ?

You get enough natagama from ninja exam. Unless your playing on a new server and trying to get top 20 power, I wouldn't bother. Save your coupons for certain events that interest you or help you directly. Things that are hard to get are mood scrolls and medium threads at the moment. To be honest I dislike rng, even if I get a good value, I dislike the randomness of the rewards. Its your coupons though. If your going to buy dice, wait until the last day of the event and buy as many as you need. Nothing worst then overbuying and wasting your precious coupons.

You do NOT get enough magatama from Ninja Exam!

You should invest in Lucky Dice. This is the first event in a long time that's actually worth it and I believe it may be the most efficient in terms of coupons/ingots spent for rewards. You will get much more value than 400 coupons. Likely 2-2.5x as much. Nobody is dropping money on any other part of the event like they are for the Lucky Dice because Lucky Dice has the best return.

I got lucky with Hashirama in 60 boxes YEEEEEE

My vote on Lucky Dice as well. You get more investing in it that anything else. Every time you land you get one mag, seal scroll, summon scroll, or etc. and when you pass Start/Go you get a Ninja Box. In that box even more goodies.

Lucky board give you more good stuff

Well, i had no luck with the "landing"(expect like 3 summoning scrolls and 3 seal) but from the 9 lucky ninja boxes i got 3 lvl 5 magetamas and one lvl 4, spend like 900 coupons

Lucky board as you stand a chance to gain more goodies.

Thanks guys I'll do that

Spent 1600 coupons, only 1 level 5 magatama, rest were level 4 :'(. But even still the value was so worth it from all the other goodies I got.