The best and least expensive way to get 25 frags of rinne sasuke is on naruto's ramen party.

the points you need to get a fragment is 22 points. So you the points you need to get 25 frags is 550.

Each ramen can earn you a point. So you need 550 ramens to get the 25 frags.

You can earn the ramen you need by clearing plot/elite instances or buying it.

If you buy the needed ramen you will need 5,500 coupons to get all 25 fragments you need.