Basic Engineer strategy
From Team Fortress Wiki
The Engineer has the unique ability to construct Buildings. Although he possesses a weaker set of weapons, his Sentry Gun is more than enough to fend off multiple enemies. His Teleporter system sends teammates to the front lines, while his Dispenser heals and supplies ammunition. After constructing a building, he can pick up and haul it to a new location, allowing him to keep his base alongside an offensive push. The Engineer and his buildings will frequently be targeted by Demomen and Spies. A well-timed enemy ÜberCharge, in particular, can quickly ruin all of his hard work. When his buildings are destroyed, it is most important that the Engineer makes it out alive so that he can reconstruct his base. A good Engineer will place his buildings in strategic locations and not be afraid to get his hands dirty defending them.
The Shotgun is a deceptively powerful weapon at close to medium range. Use it to defend yourself when alone and scare off enemies that your Sentry Gun cannot target, such as disguised Spies. Out of your available primary weapons, the Shotgun deals the most consistent damage.
Barring magazine size, the Frontier Justice is statistically identical to the Shotgun. You'll get the most of the Frontier Justice if you place your Sentry Gun or Combat Mini-Sentry Gun in an offensive position, where it should be able to get a few kills or assists before being destroyed and granting you Revenge crits. Even if you have stored up a great number of Revenge crits, the small magazine size in addition to the threat of losing all of them upon death will prevent you from going on a mini-rampage. Although you can destroy your own Sentry Gun and still receive Revenge crits, the firepower of your Sentry Gun is generally more valuable.
The Widowmaker consumes 30 metal per shot, but will refund metal based on how much damage you deal. Accurate shots at close range will usually yield far more than 30 metal. Medium to long range shots, however, will generally yield close to or below 30 metal. Unless you have good accuracy, you'll generally be inefficient if you fight at the frontlines with this weapon and will often find yourself out of metal. It's best used when defending your buildings, as your Dispenser can replace most of the metal you lose from firing the Widowmaker at a distance. Equip the Pistol alongside the Widowmaker so that you have a reliable ranged attack even when out of metal.
When compared to the Shotgun, the Pomson 6000 is less effective in direct combat; it deals reduced damage at close range, has a smaller magazine size, and its projectiles are slower than standard bullets. However, it is accurate and reloads quickly, so you can use it to harass enemies at a distance. The Pomson 6000's true value lies within its utility; a Medic or Spy shot by it will lose a small portion of his ÜberCharge or Cloak meter, respectively. This can be used to prevent them from attacking or escaping your team, particularly when dealing with the standard invincibility-granting ÜberCharge or Spies using the Dead Ringer. Consider using the hitscan Pistol alongside it so that you have a more reliable weapon for self-defense.
The Rescue Ranger grants you the ability to haul buildings at range. This ability has an expensive cost of 130 metal; if in combat, prioritize preventing your Sentry Gun from being destroyed. Long range hauling requires a direct line of sight, meaning you can safely move buildings across open areas and save them from being destroyed, but can't sneak them into the enemy base. Hauling a building will mark you for death, encouraging you to only use the hauling mechanic when out of combat. The Rescue Ranger fires weak bolts that can safely repair friendly buildings from a distance, allowing you to save metal for an emergency long range hauling. This is balanced by your capabilities at close range, where the Wrench repairs faster and hauling is free. Overall, the Rescue Ranger is an excellent weapon if you rarely use the Shotgun and prefer to rely on your Sentry Gun for firepower. Its mobility pairs well with the Jag, which aids in redeploying hauled buildings. The Rescue Ranger doesn't work with the Eureka Effect, which removes your ability to haul buildings.
Pistol + reskins
You have a tremendous supply of Pistol bullets, so feel free to spam them to fend off enemies, check for Spies, or protect your Sentry Gun. Its effectiveness at medium range makes it combo well with Combat Mini-Sentry Guns.
When the Wrangler is your active weapon, your Sentry Gun will gain a shield that reduces damage taken and can be aimed manually, even if it's not within your vision. While under manual control, your Sentry Gun can fire bullets and rockets more rapidly than normal, but will not automatically detect targets. Manual control is particularly useful for attacking enemies outside your Sentry Gun's normal range. Combined with the Wrangler's shield, you can lay siege on any visible location. However, you'll typically need to expose yourself to aim the Wrangler. Deactivating the Wrangler will disable your Sentry Gun for 3 seconds, so you need to balance its activation with Sentry Gun repairs. If any enemies manage to get near you, you'll either have to switch weapons and temporarily fight without your Sentry Gun or use the Wrangler at close-range and risk shooting yourself. If you need to focus on something else, don't be afraid to allow your Sentry Gun to function normally; its AI is more aware and accurate than you are.
The Short Circuit fires a wide arc of electricity that deals minor damage but can destroy enemy projectiles. It's particularly useful for defending your buildings or teammates from rockets, grenades, and even Stickybomb carpets. Because it draws from your metal supply for ammunition, you should stay near an upgraded Dispenser so that you always have a shot to fire.
Wrench + reskins
Hitting buildings with your Wrench will cause them to build twice as fast. When playing as the Engineer, the game prioritizes repairing buildings over melee combat; if you attack an enemy Spy who is standing next to a friendly structure, you will hit the building instead of him. Use your Shotgun, which deals greater damage, to dispatch nearby foes instead.
The Gunslinger grants you additional maximum health and replaces your Sentry Gun with a cheap, rapidly constructed Combat Mini-Sentry Gun. The Gunslinger is great when constructing a full Sentry Gun can be inefficient, such as when you're on the offensive, in the middle of combat, or moving around too often. If you're staying at medium range, support your Mini-Sentry Gun with the Pistol or Wrangler. For a more aggressive playstyle, use the Shotgun or Widowmaker; the Shotgun needs to be reloaded less frequently while a few well-aimed shots with the Widowmaker will give you enough metal to build another Mini-Sentry. While the Gunslinger can deal a guaranteed Critical hit upon landing three successful consecutive attacks, using a loaded Shotgun is less risky and deals more damage-per-second at point-blank range.
The bleed effect that the Southern Hospitality inflicts can be useful in a one-on-one fight and helps to track down Cloaked Spies. The fire vulnerability is negligble; if a Pyro manages to catch you alone and away from your buildings, you're probably going to die anyway.
Hitting buildings with the Jag will cause them to build even faster than when using the standard Wrench. This trait makes it useful for quickly constructing or redeploying a nest of buildings if you find yourself constantly moving around or need a Sentry Gun immediately. The Jag deals less damage than the Wrench, so use your Shotgun against close foes instead.
The Eureka Effect allows you to use alternate fire to teleport back to your current spawn room at the cost of no longer being able to haul buildings. You can combine its teleport with your own Teleporter system to instantly restock your metal supply, particularly useful when you're constructing a fresh set of buildings. Additionally, if the teleport's moderately long animation isn't interrupted, you can use the Eureka Effect to escape dangerous situations. To compensate for the inability to haul buildings, you'll either need to use your teleports to quickly rebuild bases or ensure you're already camped out where you want to be.
You are most valued for your Buildings, which all have a sizable impact in a battle. It takes time to fully set up a base, which means you need to choose how to spend your time accordingly. Because the Sentry Gun provides most of your firepower, you'll want to fully upgrade it first in most situations. If your teammates can hold the line without the aid of your Sentry Gun, consider upgrading your Dispenser or Teleporter system first to further aid them. If any building is in danger of being destroyed, you can haul it away; it's sometimes more important to save an upgraded Sentry Gun than to stall your enemies.
It's best to place Sentry Guns in well-frequented areas to prevent enemies from passing entirely. When placing a Sentry Gun, note where it can shoot and where the enemy can fire from. If a Sentry Gun is placed out in the open, it can cover more ground but will be left more vulnerable to attackers. If placed in a corner, it is naturally protected from flanking by walls, but can only focus on a certain angle of the battlefield. The difference between excellent and average Sentry Gun placement can be very small. For example, moving it even a few inches can keep it from being picked off by Soldiers or Snipers. If you place your Sentry in a more unexpected position, such as looking down holes in ceilings, narrow hallways, or around sharp corners, make sure to compensate for what the Sentry cannot reach with your Shotgun.
Due to their small size and quick build time, Mini-Sentry Guns can often be placed in positions that a normal Sentry Gun cannot and perform exceptionally well when laid in concealed areas. These advantages, coupled with the bonus health the Gunslinger provides, allow for relatively aggressive play. They only cost 100 metal to deploy, so you should be able to provide a constant source of suppressive gunfire. Don't hesitate to place them out in the open as distractions or to cover a retreat; players tend to fire at them before you. They are far less useful when placed in defensive positions, where their reduced firepower won't be able to fight enemies off.
A Dispenser is typically the first building you want to construct so that you have a stable source of metal. Placing Dispensers in forward locations will keep teammates healthy and allow them to hold an area for a long period of time. Additionally, because the Dispenser is a solid object, you can hop on your own Dispenser to climb to inaccessible places or block narrow passages from enemy Spies.
Always have a Teleporter system active to help a steady tide of reinforcements reach the front lines, even if the distance covered is short. Rotate the Exit so that it faces away from a wall and allows teammates to see the battlefield immediately upon coming through. Be aware that the Teleporter Exit's placement can impact the flow of battle; sending teammates to poor locations will weaken both your offense and defense. If either Teleporter member comes under fire, hitting one end with your melee weapon will repair the other. If guarding your buildings, refrain from standing on your Teleporter Exit, as enemy Spies are able to come through and telefrag you.