A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Formations are not in armies, so nothing opposes them. No.
Flank Formation • Formed by 2 Shadowlands Personalities. Battle: Target and bow a card you control at the current battlefield that this Formation has not targeted: Either straighten a target card you control, or destroy a target opposing card with no attached cards.
I think you need to revisit this answer.
The formation does not indicate that the target of the destruction has to be a card opposing the one bowed to pay the cost.
I think you need to revisit this answer.
The formation does not indicate that the target of the destruction has to be a card opposing the one bowed to pay the cost.
"Destroy target opposing card" means "opposing the card you bowed." Opposing means opposite army, which means your card must be in an army. Armies are made of units. A Formation is not in a unit. It's not in an army. It cannot oppose anything. It cannot be used for this action.