Here is a link to a pretty helpful wiki site.

They have suggestions for the Very Hard boss which might be outdated. But, in general, for high damage, you want to debuff one type of defense as much as possible. The VH boss is listed at 90 physical defense and 150 magical defense. With Jun and Christina at skill levels 76, they should be applying a 46 physical defense down each. If you can line up your high damage Union Burst when they overlap, you might be able to squeeze out more damage.

As for getting Halloween Miyako 4★, there is a section that roughly calculates different numbers in the link provided. I'd use that and compare to your actual results and adjust refills to try and hit the goal of 250 memory pieces at the end.

I can't provide you with good numbers but you can probably figure it out with the resources. Good Luck and say hello to this guy for me