Hidden Extra 1
When you first start in the asylum, you can see it through the window in the next room over. Go down the hall, get the gun, and kill the Deadite. Go back and target the window. Shoot it out and you can climb through and get it.
Hidden Extra 2
In the asylum, where you have to shoot the flying furniture then get on the elevator, there are two bathrooms that you can go into and open the stall doors. Open the doors to find it.
Hidden Extra 3
In the asylum, after you electrocute the man on the table and are heading down the stairs, there will be an open door on your right.
Hidden Extra 4
Get Sam and enter the graveyard. After the first group of Deadites come, there is a break in the fence. You can go into the water, and far back is the hidden extra.
Hidden Extra 5
After the graveyard save point, when you hear the loud footsteps just before you learn to take possession of Sam, on your left is a stream. Go into it and back a little way.
Hidden Extra 6
After you change into Sam for the second time (after feeding the souls to the Soul Eater), go down into the cave. Go into the stream. The current will take you to it, but Sam will get killed on spikes.
Hidden Extra 7
In the mine level on the platform where the second spirit you have to collect is found, go to the right (in the opposite direction of the egg).
Hidden Extra 8
In the Woods level, you will reach a clearing where you will then be attacked by Deadite Dogs. After you defeat them, you will see a bridge that is up on a ledge and. To left of it is the Hidden Extra. You must progress through the level until you reach an area where you must ride a Deadite with Sam. After you kill it, you will encounter the Harpoon Gun in a mining area. Fight your way back towards the clearing, and before you reach it, there will be two small cliffs you can climb up. Once up on the ledge, target the hole at the top of the bridge with the Harpoon Gun and it will bring the bridge down. Go across to collect the Hidden Extra.
Hidden Extra 9
In the mining area, after defeating the flying Deadites, to your left will be a building and two crates that you can climb up on. Once on top of the building, you can target a bridge with your Harpoon Gun. Once it is down, cross over it and target the mining car. It will be pulled out of the way, revealing the Hidden Extra.
Hidden Extra 10
At the start of the Forsaken Mine level after the intermission sequence, there will be a mining cart to your right. Target it with the Harpoon Gun and pull it out of the way to reveal the Hidden Extra.
Hidden Extra 11
When going for the Second Spirit in the Forsaken Mine level, instead of going to the egg, turn to the left to collect the Hidden Extra.
Hidden Extra 12
In the Shipyard level, after getting the rocket launcher and after the save point, you will encounter a group of fat Deadites. The handgun is best for killing them. After killing them, enter the blue crate on the left.
Hidden Extra 13
After riding the Deadite in the Shipyard level and being ambushed in the warehouse, climb up the crates and go through the door. Go down the walkway and on your left there will be a sign that reads “No Smoking”. Down from that after the building, turn left and go down the walkway to collect the Hidden Extra.
Hidden Extra 14
On your way to getting the second spirit down the stairs, kill the big Deadite. Go back to before where you get the spirit and down by the semi containers. Beware of a Deadite ambush.
Hidden Extra 15
Posses Sam in the swamp. You will have to again ride the Deadite. Around the swap are little mounds of trash. Get on the Deadite, go out of the fence, and smash the trash on your left.
Hidden Extra 16
After you open the gate after defeating the big Deadite as Ash, go into the swamp. Go under the bridge, and past the ladder on the right is a cave which you must open with the rocket launcher
Hidden Extra 17
When you enter the town, go down the main street. On the right is an alley. Go down it.
Hidden Extra 18
In the area where Sam needs to open the warehouse door, walk around the building.
Hidden Extra 19
At the start of the Alternate Demontion, turn right and follow the path.
Hidden Extra 20
When you finish the arena, go around the walkway.
How to defeat Frog Deadite Boss:
When the Frog Deadite goes to impale you with its tail, move out of the way and it will get it stuck in the ground. Cut it with the chain saw so that the Frog Deadite opens its mouth. When the mouth is open, fire a rocket into it. It should take five or six rockets to bring the Boss down.
How to defeat Rhineheart:
When Rhineheart comes at you in the final level, start firing your missile gun immediately. During some periods he will shoot ground level blood lines. Just dodge them. Other times he will shoot full circle blood waves. Jump those. If he gets close enough to you, he will pick you up and start punching to severely drain your life. When he gets low on health he will run to the altar to recharge. If he recharges you must repeat the process all over until he is dead. Face the altar when he picks up the book with his powers to recharge. Kick Sam towards the book, unhooking Rhineheart from recharging. Repeat the missile launching and dodging the blood lines and blood waves. Remember to stay away from him. Repeat kicking Sam towards the book. When he is low enough on health he will basically slowly walk back to the altar. This should give you enough time to finish him off.
Shake the screen:
When you exit the furnace where Sam is shot out of the smoke stack, aim for the screen as you are walking. Sam will hit it and the screen will shake.