For my players this was actually a very hard fight.
The party consisted of two warriors (shield and bastard sword and two handed fighter) a wizard, a inquisitor and a rogue. They went through the lopper, the mosswater marauder the maiden and the hands and the gary ooze in a single session before meeting the splatter man...
The haunting almost got the rogue unconscious (wisdom 7 :P) and they provoked the collapse (the two handed weapon fighter hitted really hard the walls, I used hardness 10 as they were in a very bad condition) only the rogue avoided the damage and that left them in a really bad shape when the splatter man appeared from his cell...
Only summoned the rats once and used the magic missiles dividing them among the players (it was funny, they usually kept themselves separated enough so he couldn´t target them all with the missiles so several turns some of them were hit by two or three missiles, ouch)
I kept moving the splatter man from one place to another, even provoking opportunity attacks, using the dense terrain to keep the melee PC´s away.
Meanwhile the wizard had the feeling in his guts that ripping the book would harm the splatter man (Some cinematic vision I gave him, he suffered the vision of his name being written in his room at kendra´s home) but he avoided doing it because he wanted to keep the spells in it! Finally he started to do it... with great pain.
Finally they managed to kill him with some nice throws and consuming almost all the healing potions they found.
In the end all my players were amazed with the adventure. They said it was one of the best the´ve ever played and really enjoyed playing through it.
We are actually taking a break from Pathfinder as one of my players is DMing a Deathwing campaign and another one plans to DM a couple of sessions of Star Wars, but they all agreed to play Trial of the Beast to see how the story develops.
PS: Ok, I´m actually from Spain, so my English is not as good as it should be. So, excuse me for any mistakes writing this. Just don´t poke out your eyes :P