Shadow Queen sucks.

Final Boss:
Palace of Shadow, Coffin room
Shadow Queen
HP: 150 ATK: 7 DEF: 0
Description: This is it. The final battle. To not make a mistake that
I made on my first time, don't use any special attack other then Sweet
Feast, no matter how desperate you are for a multi-target attack.
When the battle starts out, the Shadow Queen will mostly use a
Lightning bolt attack. It's pretty fast, and not easy to dodge.
If you have a lot of Pretty Lucky Badges, wear those. You wont get attacked
By the lightning as often. Since most of her attacks are hard to defend, this
Will last you a while. Also, you will not regret using your Feeling Fine and
Feeling Fine P badges.
Her other attacks will bring you down into the shadows for 3 unblockable
Strikes. And she can boost her ATK and DEF. (Big surprise there.)

After half of her HP, she'll change into her original form. Here, she'll
Have two large hands that attack you, so that's 3 hits on you per turn. The
Hands will either slap you from above or suck out your HP. If Mario has
A lot of Pretty Lucky Badges like I suggested, then you'll find that you
Dodge the attack where the hand absorbs your HP often. Use a multi-target
Attack such as Fiery Jinx from Vivian, or Koops' Shell Slam, to get rid of
The hands. If they morph into the dead hands, you wont be able to get rid
Of it's 8 HP by using a multi-target attack, so use a strong single target
Attack. Devote one character to destroying the hands and the other to
Damaging the Queen.

She'll also have status affect attacks, so be ready. Continue attacking the
Queen for 4 more turns and you'll trigger an insanely long and annoying
Cutscene where the Shadow Queen gets full HP. But fear not! After this long
Cutscene, your HP, FP, and star power will go set to the max!

After this, you'll be starting the fight again, with you and the Queen
At full health. I described all of her attacks in the paragraphs above
This one, but this time she also gets all shiny and gets +7 ATK for a
Turn. If this happens, you should keep Vivian alive and use her Viel
Power to avoid the blow. The large blow of darkness she throws out
After charging up does a lot of damage, and scares off some of your
Audience. Because of this, you wont want to have to rely on your star
Power a lot for this fight. Only use it for emergency Sweet Feasts.

Also, if you don't want to waste a turn using Vivian's Veil, you can
Use Koop's Shell Shield to protect Mario, or put someone in front to
Use as a tank. (For example, but Bobbery in front so that he takes the
Damage, then switch him out next turn. This is useful with Bobbery
Because of his large max HP.) I'd recomend the Shell Sheild idea the
Most, but you could also use Flurrie, since she can recover her HP and
Attack at the same time with her Lip Lock attack. And if you're feeling
Really lucky, you can try using Flurrie's Dodgy Fog attack.

Sorry that was supposed to be quoted from gamefaqs.

Tap b as much as you can when the shadow queen attacks you.

No, don't tap B - Button tapping kills your dodge.

Most people clear Shadow Queen the easy way: Danger Mario. (Lowering your HP to 1 and equipping lots of badges that increase your attack power when you are low on health) However, if you can't do that, you can do it otherwise.

The start of the battle isn't that bad. You can attack, dodge, and heal, as normal. Try to preserve your items on this part of the battle.

Afterwords, she will show her "True form" and you will now have to deal with her hands. Usually you can do that by using a multi-hit attack (Shell Slam, Power Shell, Fiery Jinx). But to actually kill the hands takes attack-boosting badges. Don't do Danger Partner here though, you need to save on your partner's HP, so I recommend Attack Plus P badges.

Eventually on the battle, she will charge up an attack. If you don't dodge it, that's 15 damage on you!

Later she'll eat up your audience. So keep your Star Power for emergencies, you'll need to heal often.