Once you’ve beat some bosses you can use the Dream Nail on them and fight them in their Dream Boss form where you gain essence for beating them. In general, the moves are the same but the fights are much faster.
Here is a video guide to all the Dream Boss locations.
Grey Prince Zote
Failed Champion
Lost Kin
White Defender
Soul Tyrant
The Soul Tyrant is the Dream Boss form of the Soul Master. It is found in the Soul Sanctum where you beat the Soul Master.
The build above relies on the Weaver Song Charm