This is a collection of maps, arranged in a campaign, demonstrating ways to detect the player's skill using vanilla Half-Life setups.
The first map has the setup for detecting the skill level, then actions based on the result including removing entities, and setting some globals.
The second map reads those globals value and does things based on the values read. This includes only having level changes to one of the remaining three maps based on the skill level.
Read the included README.txt file for more information.