There are just two maps for the battle. Both are very easy, though RNG made me run one of them twice. Just go to your unit creation screen, select an unused group of teams, and fill by selecting 'Blossom Hill.' Then adjust your unit composition to spread out your girls a bit over the available teams. I wasn't able to fill every spot, but a team with 4 is more than enough...you're probably good with even 3 or 2 per team.

Just make sure you select that unit after you choose your helper (and select another unit when you're back to your normal maps).

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OP: It also seems like the helper squad doesn't have to meet the nation requirement.