I remapped this puzzle, its easy to build, and funy to play.
First you need a Building (I prefer one with a wedged roof, you can see why!) thats high enough that no one on its top can grab something on the ground with his Gravity Gun.
Some Spawnpoints on the Ground on one side of it, with some Gravity Guns, Rocket Launchers and Rocket Ammo.
One essential Thing is the classic "Jump on the other Player to get into the ventilation Shaft" Puzzle. Make shure that the Ventilation shaft is 64 units above the Ground, so one player have to left behind, and be sure that no Physic objects are around wich can be used to jump into the shaft!
Put whatever you want into the Building (I just did a room with a ladder to the roof) that bring the players up the roof at last.
Now there have to be a enemy which can only be killed with Explosives, a Strider, a Helicopter or a Gunship. Make sure that the Player who left behind on the spawn area isnt able to see or attack this enemy!
Now the player who left behind have to throw the RPG Missiles up on the Roof to help the others to beat the Strider, if you have a wedged roof like I did, the Rockets roll back down, so you cant just throw them up, you have to throw them up again if they fall down.
If the strider is beaten maybe a Door or something can open so the player who left behind can stick to the team.
I hope this Tutorial inspired you for some Coop Puzzles. Implement it to your map if you want to!
Heres the Example map!
Always and more creative enviroments that can interact with the player like canon.