Basic Demoman Strategy
The Demoman is a well-rounded artillery class that excels at indirect combat, crowd control, and area denial. His toolkit contains many versatile weapons that can be aimed over obstacles while staying out of sight. Due to his immense damage output, the Demoman is fantastic at fighting multiple opponents and demolishing Engineer buildings. However, he is most vulnerable at close range, where his weapons are more difficult to aim, take time to arm, and can inflict self-damage, making him weaker to classes such as the Scout or Pyro.
Primary Weapons
Grenade Launcher
The Grenade Launcher fires pipebombs in an arc, dealing 100 damage on a direct hit at any range, plus splash damage against any enemy nearby. As with the Soldier's Rocket Launcher, damage is most concentrated in the centre of the blast, and it is necessary to lead your shots and predict where the enemy will be to deal maximum damage. The Grenade Launcher will be your primary weapon during firefights and excels at mid-range. Although direct hits deal the most damage, grenades that don't immediately hit enemies will still bounce and explode after a short time, so it is still viable to spam out the enemy over a corner or at range. However, you only have 20 pipes in total before needing to scavenge for ammo, so don't spam too recklessly. The Dynamite Pack gives you +50% max ammo for your Grenade Launcher when equipped, so use that if you value mid-range spam over trapping with the Stickybomb Launcher.
Gunboats
The Gunboats trade your Grenade Launcher for -60% self-damage from your Stickybombs, which can allow you to Sticky jump far more often. However, if your explosion damages an enemy player (excluding enemy buildings), the damage reduction is ignored and the user takes full self-damage, so be careful of explosive jumping around enemies. Demomen who prefer to roam the map will find this weapon very useful for extra mobility. However, given that you no longer have a primary weapon when equipped, you will need to make up for your lack of immediate mid-range damage.
Secondary Weapons
Stickybomb Launcher
The Stickybomb Launcher is an extremely useful trapping and spam tool. Stickies attach themselves to walls and ceilings and must be detonated with your alternate fire. You can use the sticky's adhesive properties to create clever "sticky traps" for unsuspecting enemies to walk into. Stickies can also be used to blast jump if detonated under you. However, unlike Soldier, Demoman does not have a passive resistance to his own explosive damage (unless equipped with the Gunboats), so don't jump too haphazardly. Additionally, the Stickybomb Launcher is a very useful backup tool for when your Grenade Launcher runs out of ammo. They can be charged for up to 4 seconds to be shot farther distances, which can be great for sniping low-health players from range. However, stickybombs can be destroyed by any stray bullet, so enemies aware of your plans will be able to take down your trap easily. Additionally, Stickybombs take 0.7 seconds to detonate after firing, so you must detonate with good timing to deal maximum damage. The Stickbomb Launcher also has a very long reload time due to the large clip, so make sure to find good cover to reload and get back in the fight.
Dynamite Pack
The Dynamite Pack replaces your Stickybomb Launcher with a throwable cluster of dynamite sticks. Hold down the primary fire to charge your throw; the longer you charge, the farther the cluster will be thrown. Once thrown, it will beep three times before exploding and creating four small bomblets. These bomblets will detonate individually at 1-2 second intervals. The Dynamite Pack has a cooldown of only 10 seconds, making it very useful to spam at unaware enemies at mid-range or throw over cover. A cluster that directly hits an enemy will slow them down for 0.5 seconds, which can have the niche use of delaying an enemy push. However, Engineers can defuse the main cluster with their wrench, and Pyros can defuse the bomblets with their airblast, so be careful when aiming for these classes.
Mine Layer
The Mine Layer is a great "fire-and-forget" trapping tool. It trades self-detonation for automatic detonation, allowing you to defend multiple areas at the same time. You are also able to view outlines of your deployed mines through walls, allowing you to always keep track of your mines, even if you're on the other side of the map. However, you are only allowed to have 6 mines deployed at the same time, so be resourceful with how many mines your traps have and how many areas you defend at once. Also keep in mind that mines always audibly beep, and detonate 0.4 seconds after being walked over, so enemies that are aware of your shenanigans can easily exploit your traps. Pressing your alternate fire while you have mines deployed will make said mines fizzle out without an explosion. While this sounds like a negative, it can be used to quickly set up another mine trap if your previous one didn't work out.
Melee Weapons
Bottle
The Bottle functions like any other default melee and deals 65 damage. However, compared to other melee weapons, the Demoman gets more use out of it than other classes do with theirs. Since Demoman suffers self-damage from his weapons at point-blank range, combined with very long reload times for his other weapons, the Bottle is often Demoman's "Hail Mary". However, charging at the enemy head-on for a swing is not a smart move; it's recommended to bait them to get in close range before going in for the kill.