After a long hiatus (spring break!) I'm back, and in this post I'm going to address what is potentially the most powerful class in TF2: the demoman.
Originally created as a hard counter to the engineer's sentry gun, the demoman was quickly used by players to deal out massive amounts of damage in a short amount of time. If you've ever played in a 32man TF2 server, you've probably seen a demoman wipe out half of the enemy team with ease with one kritzkrieg.
We'll start the analysis of the class with its basics.
The demoman has an average standard movement speed, about as fast as the sniper or the medic. He has 25hp less than the soldier at 175 max hp, but moves slightly faster as well. Thus, he much rely more on his ability to dodge incoming damage, and has the means to do so. Oftentimes, when facing an enemy soldier or scout as a demoman, you can avoid nearly all the rockets and/or bullets of the enemy by quickly strafing left and right, as well as utilizing air control tactics covered in this link.
Like the soldier, the demoman has the ability to use his weapons to gain mobility. Instead of a rocket jump, the demoman can use his stickybomb launcher to cover massive distances. These jumps do slightly more damage than a rocket jump might, as sticky bombs do more damage than rockets up close, but they also allow the player to go higher and further than any other class. The primary grenade launcher can technically be used to perform jumps, but these are universally regarded as ineffective.
Below is a video of some jumps done using the demoman's sticky launcher.
Obviously, other players are not in this video, however, I can attest to the fact that a player who has mastered these jumps (called a "rollout") can reach the midpoint of a map like badlands even before the enemy scouts have arrived, giving him an immense tactical advantage. On a map like cp_granary, a demoman who reaches mid early can set up sticky traps at the enemy team's entrances to the mid point, or lob grenades over the crates to snag some kills. Rollouts for several maps can be found below.
This is an alternate version of the badlands rollout, done by the TF2 celebrity Reptile_. It sacrifices a bit more health for a safer, less mistake-prone rollout. In addition, if enemy scouts run straight onto the point, it gives the demoman more time and space to react to them without being in significant danger.
The demoman's weapons can consistently do the most damage per shot of any weapon, with the exception of the sniper's sniper rifle, and can deal out damage at the short to mid range faster than any class other than the heavy. He does so using two main weapons: the grenade launcher and the sticky bomb launcher.
These projectiles will explode on contact for a massive amount of damage. However, if they miss the enemy and bounce off the ground, they will only detonate after a set amount of time, for less damage, Thus, it is the primary objective of the player to hit 100% of the grenades (also known as "pipes") to do the most damage. The aiming skill required to do so is less about precision, as with the scout or the sniper, and more about predicting the enemy's movements, much like the soldier. By analyzing the enemy's movement patterns, a good demoman can "lead" his or her shots, compensating for the enemy's movement. If you enjoy physics, grenade prediction becomes a simple projectile motion problem.
Pipes also do a significant amount of damage, even though they have a lower clip size, and should never be neglected. In addition, they give the demoman a "spam" weapon--one that he can use to deter enemies from entering an area, by creating the threat of great amounts of damage.
Grenades are also the demoman's best hope at killing enemy scouts before they can kill him, especially if he is left alone. Try to predict the movements of the scouts, and keep them at medium to long range, where you can't be hurt by your own splash damage. Setting up 1 or 2 sticky bomb traps in each area you enter, before moving on, will give you another degree of safety, as most players do not expect traps to be spread out around a room.
These are the most well-known weapon of the demoman. They can do massive amounts of damage, and are unique in their ability to "trap" sections of the map. I'll cover all their uses right here.
The ostensible use of the sticky bombs is to hide them, using the environment, in strategic locations, and detonate them when enemies are nearby, as seen in this video:
Obviously, that's a bit tongue in cheek, but you get the point!
By laying sticky bombs in plain site on the ground, one creates a "carpet" trap, which is primarily used to deter enemies from entering an area. By hiding them above doorways, or immediately behind sliding gates, one can surprise enemies and get almost guaranteed kills.
Look for environmental props, such as the orange traffic cones outside the second point of cp_granary, in which sticky bombs can be hidden. These are nearly impossible to spot, and oftentimes will snag you kills on otherwise very observant players.
However, the real strength of the sticky bombs comes in their ability to deal massive amounts of damage to specific locations. By utilizing the previously mentioned prediction skills, you can wipe out groups of players with relative ease, especially when under the influence of a medic's kritzkrieg. There are scripts that exist that allow a player to rapidly spam sticky bombs by holding down both mouse buttons; however, these actually limit the effectiveness of the player, because they take away his capability to time his shots, and to adjust their distances. In nearly all cases, manual aiming and firing is superior to script-assisted firing. Aiming for large clusters of players will give the best results, as you can maximize the ratio of damage done to shots fired.
This last video is a compilation made by a high-level competitive TF2 player that was meant to showcase the skills of three demomen within the competitive community. Take a look at it, and see if you can identify what makes them so effective, especially given what you've just read!