Enemies hit really hard on ?-8 and there are a lot of them, managed to 3 stars it with a single group left. I placed the slowest squad on the bottom node and a fast one + a strong helper on the top node so the helper can take care of the enemy node there with enough damage to face the boss. Also had a lilac on the left squad but I'm not sure if it would have changed much. Squad speed: 456 on the bottom one, 584 and 572 (helper) on the top one, the other 2 squads in between those.

?-8 also seems to drop ampys like ?-4.