The game has a hard time differentiating what is rear, and it attacks the 'front' of the rear squad/units. Something I have found that works is putting my flankers further on the grid to the south or north (Example shown below where I'd put something I wanted to flank and attack rear, of course this doesn't work if he has something on his flank attacking)

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I think that units or squads have a "if something gets within XYZ amount of squares, attack it" type of behavior. So if you keep them as far away from other units as possible they will get a better shot at going what you are hoping for (Archers, Rearmost).