When you reach the part in the 'No Mercy' level where you get to the lift and it calls a horde go all the way to the left of the lift instead and there will be a garage door a different size. If you keep hitting it it will eventually break leading you into the next area allowing you to skip the lift.
At the main menu select 'Options' followed by 'Keyboard/Mouse' and activate the 'Allow Developers Console' option. Then during gameplay press the '~' (tilde) key to bring up the Cheat console and type 'Sv_Cheats 1'. Then press 'Enter' to enable cheat mode for everyone on your hosted server and type the following codes (without the quotes) at the console window and press 'Enter' to enable it's effect.
Invincibility: Type 'god'
Full Ammunition Type 'give ammo'
Unlimited Ammunition Type 'sv_infinite_ammo 1'
Gives you the SMG: Type 'z_spawn weapon_SMG'
Toggles No Clipping Mode Type 'noclip
Be Spectator: Type 'ent_fire !self setteam 1'
Be Survivor: Type 'ent_fire !self setteam 2'
Be Infected: Type 'ent_fire !self setteam 3'
Quick Change Map: Type 'changelevel (map name)'
Display Available Servers: Type 'openserverbrowser'
Enable Internet LAN (enable when at 'Campaign Lobby' screen): Type 'sv_lan 0'
Start LAN server (enable when at 'Campaign Lobby' screen): Type 'map (map name)'
*Spawn Indicated Item: Type 'give (item name)'
**Spawn Indicated NPC; Type 'z_spawn (NPC name)'
*Item Names Type one of the following with the 'give (item name) code: