You can access Bestiary on your Function Menu next to Pet icon

Right now there are only a few instances and their bosses covered

– Frozen Hell, Dawnlight Halls and Uncharted Paradise.

Under each instance, you will be able to view the bosses, their mechanics, and any additional monsters attached to each boss where applicable.

You can select the difficulty by picking from the drop-down menu on the top left corner of the window.

Message Board Feature (Not Available Yet)

Click the Message Board button at the top left corner of a monster’s section.
A new window will pop up where you can leave messages or tricks regarding the Boss.
Keep in mind that there is a requirement to post messages, as shown in the image.
This feature will help the community more about dungeons and boss mechanics.

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< Magic Kingdom Patch notes

Reference: gamepedia