Go digging in Bikanel Desert. You'll need to stock up on entry coins. When playing aginst Shinra, use one that allows echo bonuses.

I've found that you have to strike a balance between removing too many and too few border coins. Generally a pretty good number of coins to total up is three every round. That keeps the math simpler and building coin echos is fairly easy.

IIRC Shinra gets 1 as the core number fairly often, which breaks echos. It sems to be random, so luck plays a part.

I played Sphere Break enough to win all the rare items. It's fun once you get the hang of it.