Play the Nazi Zombie level without ever finishing the game and add in the game cheats such as 'God', 'Give all' and 'ufo'. With the ufo code in Single Player mode you can go outside or on top of the house.
At 'Game Options' select 'Yes' for the Game Console. Then during the game press the '~' (tilde) key to bring the console up and type the following codes (without the quotes) to play the corresponding level.
Play 'Eviction' Mission: Devmap ber2
Play 'Semper Fi' Mission: Type 'Devmap mak'
Play 'Little Resistance' Mission: Type 'Devmap pel1'
Play 'Their Land, Their Blood' Mission: Type 'Devmap see1'
If you need help in this game press the '~' (tilde) key during gameplay to display the console window and then type 'devmap mak' and press 'Enter'. Doing this will enable 'Cheat' mode allowing you to type the following codes (without the quotes) to activate the corresponding effect.