How does one actually do this? After numerous trial and error attempts I managed to survive all the way to the lever with the Manticore in the poison trap room, and it didn't do anything. He just stayed alive in the room with his face poking through the door.

I am level 5 at the moment, is that the issue? I honestly cannot figure this out, either he one shots me with Spikes before I even reach the lever, he doesn't follow, or the one time I make it he just doesn't die.

Serious plea for help, thanks in advance!