Basic Pyro strategy
The Pyro is a high damage, close range combat class who excels in dealing large amounts of damage to groups of enemies very quickly, thanks to his Flamethrower, which shoots flames that pierce through enemies. Between that and his limited range, he is often considered to be an ambush class, able to wipe out large groups of enemies when he catches them unaware, with afterburn being able to finish them off if they come close enough to death. In addition, his Flamethrowers airblast allows him to reflect projectiles for extra damage, extinguish friendly players, and knock enemies players back to make follow up shots with his secondary much easier. His afterburn makes him exceptional at catching and tracking enemy Spies, but his limited range makes him weak to Scout and Heavy.
Primary Weapons
The Flamethrower consists of three main mechanics: Afterburn, Airblast, and the fire itself. The fire itself comes out in a short, straight stream of lingering flame particles that gain damage the longer they are in the air. As a result, an effective strategy for Pyro is to maintain distance and strafe when attacking with fire, aiming where the enemy is about to be rather than where they are. Airblast is where its mechanical skill mostly lies. Airblasted projectiles deal mini-crit damage, allowing the Pyro to turn the tides of a fight in his favor with a well-timed reflected rocket or grenade. A skilled Pyro can reflect projectiles on instinct, frequently extinguish enemies, and even counter ubers by airblasting Medics away from their patient. Afterburn is a mechanic where, after lighting an enemy on fire with either the Flamethrower or the Flare Gun, the enemy will continue to take 6 damage a second for 10 seconds unless extinguished. This is useful for not only dissuading enemies from fighting a Pyro, as they could die even if they kill him, but also for a Pyro to use hit and run tactics, dealing large amounts of damage to a group of enemies before running away, leaving them hurt and dying.
Secondary Weapons
The Shotgun is your stock secondary, and for good reason. Bog-standard and simple to use, this tool is useful for finishing off enemies outside your Flamethrower range, getting the edge in Pyro vs Pyro engagements, and dealing quick, reliable burst damage to enemies. It is a solid, reliable choice in all situations.
The Flare Gun trades the reliable damage of the Shotgun for fast burst damage that's good at any distance. The Flare Gun inflicts afterburn on enemies it hits, while also dealing critical hits to burning enemies for a total of 90 damage at any range. Its passive reload, fast deploy speed, and increased range makes it a strong and sturdy competitor to the Shotgun, especially for Pyro players who prefer to get quick, clean kills. One common use of the Flare Gun is to set an enemy on fire with the Flamethrower. airblast them upwards into a wall to ensure they cannot escape, and then pull out the Flare Gun to finish them off with a critical hit. In addition, hitting flares on a burning Heavy can allow Pyro to win a fight so long as there is a corner to hide behind and the Heavy has the Minigun equipped, and hitting a flare on a jumpy [[Scout] can turn a losing fight into a victory very quickly.
The Twin Barrel forgoes the reliable damage and range benefits given by the Shotgun and the Flare Gun, instead opting for a medium damage, high spread single shot weapon that deals increased knockback both to the Pyro as well as anyone hit by it. This makes the Pyro incredibly mobile, allowing him to chase down enemies while knocking them away, forcing enemies into an engagement. Due to the damage, mobility, and passive reload, it's an effective choice to get in and out of situations quickly, as well as making it much easier for him to flank enemies.
Melee Weapons
The Fire Axe functions like any other default melee and deals 65 damage. Due to Pyro already excelling at close range, the Fire Axe is mostly equipped to avoid the downside of the Harvester. It is most useful for either getting ambushing other players and hoping for a random crit to insta-kill them.
The Harvester is a scythe that heals the wielder 10 hp per second per enemy (or corpse) on fire. Each tick an enemy is fire and the Harvester is held out, a heat meter is charged. When the heat meter is full, the Pyro gains a guaranteed critical hit, which will grant him full health and overheal when an enemy is hit by it. However, he will not be able to gain health with it normally. The trade-off is the extinguishing of all enemies after the Pyro dies, which can cause enemies to be more bold and aggressive when fighting the Pyro, or for the Pyro to lose out on post-mortem afterburn kills. Harvester Pyros, as a result, often play a very hit-and-run playstyle, setting groups of enemies on fire and then escaping with the healing from doing so, and ambushing enemies with the critical hit when the heat meter is fully charged. Common strategy is to use the crit to get a quick, clean kill on an unaware enemy, or to use it to quickly gain an edge against a Medic and his patient by either instantly killing the Medic, or by dealing enough damage to his patient that you can finish them off with the Flamethrower.