Remove all arts except Shadow Eye on Shulk, and Guard Shift on Reyn.
This is because you want them to auto attack all the time, but you can't remove all arts (you need at least 1).

Do this as long as you want, you will miss everything and raise affinity really easy.
If Shulk or Reyn go down the hill to attack (sometimes they are that stupid), just call them back and try all 3 to bunch up.

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