When you reach the Crypt in Haunted Hill, you must solve a puzzle involving the tombs. Once you do that and proceed to the next room, a mini-boss will appear.

You can actually kill this guy without even shooting, which is great if you're low on ammo or playing in Murder of Crow's difficulty.

1) Once the mini-boss appears, keep your distance and wait for him to follow you.

2) Once he reaches the closest tomb, circle around it and press the lever, which should bring down the metal door and instantly kill him.



This counts towards the "Thorough" achievement too!