You just have to keep hitting those combinations of 1 Core Sphere and 2 Border Coins to increase the Coin Count Echo - further multiplying your quota result. Easier said than done though. Which is why I used a program composed by Joshua Barratt to aid me through it all.
Link: -
http://serialized.net/archives/000002.html
This program is effective for beating Shinra in Chapter 3, I have not tried it against him in Chapter 5 so I'm not absolutely certain if it's reliable for him then. However, when I beat him, I did achieve the target quota in just 5 turns out of a possible 15 turns, but that's without ever bumping into a Core Sphere of 1 obviously...