From StrategyWiki, the free strategy guide and walkthrough wiki

This walkthrough will guide you to completing each map. The focus of the walkthrough is on the Ultra-Violence difficulty, when starting with the basic pistol and 50 bullets; while going through the campaign allows you to carryover weapons and ammo (allowing you to skip portions of the walkthrough), a common standard for playing this game is by individual maps with only the basic equipment, as established with the various speedrunning competitions with this game.

The walkthrough covers:

• The basic path from the beginning to the exit
• Any shortcuts that you can use
• The secrets within the map
• And changes in multiplayer games (whether deathmatch or co-op).