Engineer

The Engineer is the backbone of the team-- if he isn't doing his job, the rest of the team can't do theirs. Thankfully, as long as your teammates know which end of the gun bullets come out of, you usually won't have to worry about doing too much besides taking care of your buildings and replacing them if one of them goes down. The dispenser and teleporter are absolutely essential for your team's ability to keep up the pressure and keep the front lines still, and your sentry gun is excellent for a variety of purposes, but primarily for defending the bomb and picking off stragglers who slip past your teammates. The primary skill of playing Engineer mainly comes down to being able to quickly respond to your team's needs with your buildings, and to stay alive long enough to fulfill them.

The sentry will be threatened by Sentry Busters who will home in on the building or you if you're carrying it, and will detonate after an audio cue and crouching animation. It can be avoided by simply moving yourself and your sentry away from the buster as it's about to explode. You have to be careful that it doesn't detonate near your teammates!

As a side bonus to his regular duties, the Engineer can be a remarkably consistent and easy candidate for tank busting, thanks to his Widowmaker.

What are the buildings used for?

The teleporter allows your team to quickly reach the front lines from their spawn, and the exit should generally be kept in a static spot where it's easy for your team to get where they need to be as soon as they respawn. Ideal spots for this purpose include the roofs of the central buildings on the Gear Grinder maps, the upgrade station or clock tower in Rottenburg, or next to the silver barrel on Mannhattan (where the dispenser typically goes). After a few waves, you might be inclined to get a two-way teleporter to allow your team to rapidly go back to the upgrade station to get more upgrades or crit canteens. If you do, move the teleporter exit closer to the front lines, but placed carefully in such a way that it won't get trampled on by the robots.

The dispenser is your bread and butter as an Engineer in MvM, and is the primary reason why he's so highly favored. By placing it near the front lines and upgrading dispenser range, you can keep your teammates' ammo reserves full so long as they're close to the dispenser. Though many players will simply keep it in a static spot throughout the whole game, it's important to instead react to your team's movements-- if they start spawncamping more aggressively, if they're pushed back by the robots, or if they otherwise need to relocate-- make sure to move or replace your dispenser accordingly, so that they don't have to worry about running low on ammo regardless of where the fighting is going on. Just to stress again: If your dispenser is down or isn't near your team, you need to fix that issue as soon as possible.

The sentry gun is the most immediately appealing aspect of playing Engineer to a new player, since it gives a relatively solid amount of practically guaranteed damage thanks to the sentry's perfect aim-- but this is only as long as the sentry is consistently on the front lines. Keeping the sentry gun close to the robots' spawn is usually feasible on Two Cities maps, but tends to be problematic on Gear Grinder maps for a number of reasons. It's true that the sentry gun can be good at cleaning up small robots and for fighting giant robots, but its DPS has a hard limit and it can only fire upon one robot at a time-- thus, it should not be relied upon as the team's primary damage dealer. The sentry gun can fulfill a good amount of roles, which will be detailed in the following section...

What you can do with the sentry gun

Defend the bomb.

Though not necessarily the most obvious, this is the most important function of your sentry. The bomb, more often than not, will get past your team, and it's far more efficient to have the Engineer worry about keeping it safe than for an actual DPS class to retreat from the front lines just to keep an eye on it. No rocket science for this one-- if the bomb is behind everyone, place the sentry on top of it.

Block giants, especially Super Scouts.

By placing the sentry in some tight doorways or hallways, or by placing it next to certain corners in the bomb path, you'll be able to block a giant from getting past your sentry. Learning to do this will take some practice, but you'll come to find that giant robots tend to turn around corners by being as close to them as possible, and you can often easily stop them from their march, even if only for a short while. This is typically used against the lightning-fast Super Scouts, but can also be used against other giants, though you should pull out the Wrangler in the latter case to help the sentry survive for longer if the giant airs out its anger against the sentry.

Use the Wrangler against giant robots.

Using the Wrangler against the small robots tends to be infeasible besides being able to take more damage, but it works perfectly against giants. While using the Wrangler, your sentry will take 66% less damage against all sources, and will fire twice as fast. If a giant decides to focus on the sentry instead of your teammates, the Wrangler can be a great way to buy your team more time to kill it, and you can occasionally kill a giant all on your own.

Common mistakes

Buying the Disposable Sentry upgrade.

This upgrade is not worth it. Ever. No, not even as a gag. It costs a ridiculous 500 credits and does practically nothing-- you can outdo its damage with the Rescue Ranger.

Not upgrading dispenser range on the first wave.

To put in other words what was already established: your dispenser is more important than your sentry gun, and the dispenser is for your team-- not you. You should always upgrade its range on the first wave, not only to make it easier for your team to stay fully stocked on ammo, but also to prevent the dispenser taking splash damage due to your allies having to stand inside of it to stay loaded.

Keeping the dispenser all for yourself.

This is thankfully uncommon, but occasionally you'll see a dope who will turtle in a corner or on a roof somewhere, keeping all of their buildings, including their dispenser, glued to them. This dope will either be yelled at by their team to share their toys with the rest of the class, or failing that, will be kicked. And they'll deserve it.

Upgrading sentry firing speed instead of building health.

Not only does the sentry do sufficient damage by default, but it also shouldn't be relied upon as a source of DPS. What it should be is a tank-- by upgrading building health, it can take significantly more attention and punishment from the robots, allowing it to be used as a distraction, and as an anchor for the front lines. Sentry firing speed isn't nearly as important for the sentry's role as building health is, and therefore the latter should be given preference. If you have building health upgraded, you can invest into firing speed, but only take one tick as the other two ticks don't work properly.

Upgrade priority

As soon as the mission starts, you should upgrade dispenser range. After you've fully upgraded it, get at least one tick of building health, and following that you may balance out getting building health, metal capacity, and wrench swing speed according to your preference. Sentry firing speed, as stated earlier, can optionally be upgraded, but only take one tick. Two-way teleporters can be good in later waves, and you should listen to your team if they ask for it on an earlier wave (as long as it's not wave 1 of a Gear Grinder mission).

If you have trouble maintaining your buildings or if you fear an emergency, you can take Building Upgrade canteens to use as get-out-of-jail-free cards. Be careful not to spend too much on them, as you might risk not being able to afford upgrades to help you maintain your stuff.

Your primary should usually be left untouched, but if you're using the Widowmaker, fully upgrade its firing speed on waves with tanks.

Weapons

Primary

Weapon	Rating	Review
Rescue Ranger	S	Being able to heal your sentry at a distance can be a good help, even if it's not as consistent as simply hitting it with your wrench. The more significant upside of the Rescue Ranger is being able to zip your buildings halfway across the map, which is excellent for if they're urgently required somewhere else or if they're about to be destroyed.
Widowmaker	S	The Widowmaker can bring to the table some additional damage against giants, but more importantly is phenomenal for destroying the tank. By fully upgrading firing speed and simply holding down the fire button as you follow the tank, you can single-handedly destroy nearly all of the tanks in the game, and your sentry doesn't have to leave its post as you do so.
Pomson 6000	A*	The Pomson is good for one thing and one thing only, and that's sapping giant Medics. They can be a giant pain in the ass if you don't have a Spy or a Demoman, but simply shooting the giant Medic will quickly remove its ubercharge (ideally before it pops) and allow your team to kill it without any hassle.
Frontier Justice	C+	The Frontier Justice isn't a bad weapon, but it tends to be overestimated by new players. It's true that you can rack up a lot of crits, but you only get them if your sentry is destroyed, and you can only have up to 35 crits stored-- which you can lose if you die. The clip size and firing speed will both get in the way of it being any good against giants, but it can be used to get rid of some pesky small robots.
Shotgun / Panic Attack	D	The only thing the stock Shotgun is good for is defending yourself against Spybots and other small robots, which is rarely useful when your sentry will do the job much better, unless you're on wave 4 of Broken Parts.

Secondary

Weapon	Rating	Review
Wrangler	S+	By holding out the Wrangler, your sentry can take three times as much damage, and does double damage against giants. Great to have if your sentry is being threatened, but don't try to individually point at every small robot.
Short Circuit	A	The Short Circuit can not only save your skin, but also your team's. Being able to destroy a flurry of rockets is amazing, especially against crit-boosted giant Soldiers.
Pistol	Bad	You can still play Engineer if you don't have anything besides the stock Pistol, but it doesn't bring anything useful to MvM.

Melee

Weapon	Rating	Review
Jag	S+	Despite what some people might have told you, no, the stock Wrench is not better than the Jag. You're being fed lies by Big Wrench to jack up the price of australiums. The 20% penatly against repairing buildings only applies to their health, not the sentry's ammunition. Plus, the 15% faster swing speed just about cancels out the downside of repairing health slower. Even if you're unable to maintain your sentry's health, the Jag's faster swing speed and additional bonus for constructing buildings means you can build up another level 2 or 3 in practically no time at all, significantly faster than you would with the stock Wrench.
Wrench / Southern Hospitality	B	The stock Wrench is inferior to the Jag in just about every way, but it's still usable and viable if you don't have anything else.
Eureka Effect	Bad	The crippling slowness of constructing new buildings vastly outweighs what little advantage the self-teleporting ability gives (which is none at all). The only time when the self-teleport ever comes in handy is when you need more crit canteens to bust the tank faster, but a two-way teleporter is better for this purpose.
Gunslinger	Bad	The mini-sentry does practically no damage versus a full level 3 sentry, and is incapable of blocking giants. If you use this thing, you will probably be kicked, and I won't really feel bad for you if you insist on using it.