The shadow Queen is a hard boss so I'll help by teaching you on what's the best way to kill her.

Preparations
What you will need is atleast have 60 or more HP. You may want 40fp or more. You should also cook the the ultra shroom and Jamming jelly together to make a ultra jelly.Atleast have 2 of them. Make sure you have power bounce and Bobbery and vivan are upgraded twice. Having feeling fine for you and your partner is optional.Also briong tons of shooting stars if things get too tough

During the first phase of the fight.
This part of the fight don't use items. AT ALL.Nouthing. If your HP get's low use sweet feast. Princess peach will heal you completely after the fight and its pretty useless to use items.

During the real battle.
What you want is to use power lift in the first turn. You than attack with your partner with a move that kills her hands/little hands and hurts the shadow queen. You than use power bounce to cause massive damage. When the power lift effect wears off you. you will need to know how to perform stlyish moves.You will or have most of your star power full. When she charges you swich to vivan and use veil. When you have low fp you use your shooting stars or use a ultra jelly if your HP are low too.
When the shadow queen HP is has 15 or less finish the fight with a super nova.

Mind this don't do this step by step. This is how you can take the boss much easier and some tips.