The Eminence in Shadow: Master of Garden World Tree Floor 20

Additional tips and party structure comments below.

Nidhogg:
Shields are necessary for this boss, especially for its hidden technique. You will also need healing since it applies bleed to your highest attack characters. Having Arthur Pencilgon is especially helpful to get rid of the attack debuff the boss applies as well as increase your damage dealt since this boss has a lot of health and defense. This being said, she is not necessary, and neither is any of the rest of the team, though you may find most parties using the Aurora I used since she has a team heal with her magic item and a powerful self attack buff.
This team is taken directly from this video:    • 【カゲマス】次元超越世界樹20-1 超次元世界樹20-1  .

Mist Dragon:
This boss does even more damage than the previous, but may not need many shields depending on the team you use. For what is shown in this video, support Epsilon's SS+ magic gear is necessary to apply more shields and remove some status ailments. If you end up using a copying a party that has yellow tank Rose, make sure she is awakened enough to have her skill 2 (stun) awakened since it doubles the duration of the stun.

Malak the Malevolent:
This boss is kind of a pushover. The Shadow I used should be replaced with blue attacker Nu with her SS+ magic gear for an easier time. Just make sure you have enough survivability. Something like replacing support Nu with blue support young Beta and her SS+ magic gear might work well.

Here are a couple of videos with some different solutions to what I showed:
   • 【カゲマス】次元超越世界樹20階層(ユグドラシル20)  
   • 【カゲマス】次元超越世界樹ユグドラシル20階層(20-1、20-2、20-...  

0:00 - Party details
0:26 - Nidhogg
4:03 - Mist Dragon
6:58 - Malak the Malevolent