Find all hidden pieces and solve the puzzle before completing the game.
Play Chapter 5, "Full House". The Ted mask may be useful, as it will prevent dog attacks. You can immediately check if a brick has randomly spawned in the room directly below the entrance or in large bedroom further to the right. If it did not spawn, intentionally die and try again. Clear the floor of enemies, then get the brick. Go upstairs to force the game to save with the brick in your hand. Kill the three mobsters in the next set of rooms then approach the door on the right. When the three mobsters in this room are about to line up, enter and toss the brick at them. You must kill all three of them. If needed, use the Jake the cobra mask. Repeat the process if needed, using the saved game.
The brick spawns randomly in weapon spawn locations after it is unlocked. It usually becomes available after you unlock a hammer, when you reach 450,000 cumulative points. The Jake mask (green cobra) is the best one to equip because it gives you killing throws. It will bounce off objects and enemies, but brick must bounce off a wall and kill an enemy.
Complete Chapter 5 "Full House", then get a crowbar and go to the entrance of the level. Do not go in the car. Instead of that, walk around the house until you find a manhole located at the southeast corner of the level. Enter the manhole to find the mask next to a corpse in the sewers. Note: You may need to go to the sewers twice. Upon the second completion, "Sewer Alligator" should be earned.
After unlocking all the masks, restart a random level repeatedly, each time putting on one of the masks. You do not have to complete the level.