Splatterhouse

Story Achievement, unmissable. This is not an actual boss battle level, but a timed event. There are 3 phases to the event. I highly suggest checking out the collectibles guide HERE before doing this section so you waste as little time as possible.
Phase 1: You have 3 minutes to complete this part. Your best bet is to simply run through all the gates until you are forced to fight a few waves of forgotten with the blue veined ghouls. There is a sword in the nearby in the crates in the corner near where the waves spawn if you need it. Kill them as fast as possible, and run into the next portal when you are able to.
Phase 2: The clock will have 2:20 minutes added to it. Kick through the gate and dodge the large fish demon, running past it. After a cutscene, you will be surrounded by fishhead forgotten. Run through the gate ahead, along the pathway, into the next courtyard, kick through the gate, run the path again avoiding the fishhead forgotten at the end, and into the courtyard. Kill off the 3 huge fist demons with Berserker mode if possible, and hop into the portal.
Phase 3: Kill the fishhead forgotten and climb the wickerman. "Jen" will attack you, and you will have to shake your left stick to throw her off of you. After stomping her to death, you pass through another portal, and the level is over.