Sniper

Sniper is, without question, the best crowd killer in MvM. Explosive Headshot is by far the most powerful upgrade in the entire gamemode, and a Sniper who can consistently and quickly land explosive headshots can easily snap the game in half and carry a team. Key words there, of course, are "consistently and quickly" because you need to actually hit the headshots to trigger the massive area of effect damage. Some might be too afraid to play Sniper at all because their aim is generally poor, but the truth is that you don't need to be great at aiming to get good results in MvM, even with Sniper to a certain extent. Many robots move slowly and predictably, and don't make an effort to dodge your shots. Of course, even with this advantage, you might not have the best experience, so ultimately Sniper isn't for everyone. Still, if you'd like to give Sniper a try and see if he's for you, I would highly recommend playing the mission Caliginous Caper (aka Wave 666) on Boot Camp, where Sniper is highly favored.

Sniper's primary role, as mentioned earlier, is AoE damage. However, he is also very important for killing uber Medics, as even one tick of the explosive headshot upgrade allows you to kill them all in one shot, whether they're connected to a giant or a bunch of tiny robots. In the later waves, he can do decent damage against giants. Finally, Sniper can be a good tank buster if he takes the Cleaner's Carbine and a Bushwacka with upgraded swing speed.

Common mistakes

Not getting Explosive Headshot

This should be fairly obvious to anyone who's played Sniper more than once, but some new players will assume that Sniper is about single-target elimination than AoE damage. Because he's a pick class, this misconception could be forgiven, but it's nonetheless important to remind any new player picking Sniper in MvM to immediately put a point into Explosive Headshot, and NOT into something like damage.

Using the Huntsman

Directly tied to the previous point, the Huntsman is not compatible with Explosive Headshot and is therefore worthless. Some comedians might suggest it can do some good damage against giants, but not nearly enough to replace the sniper rifles, let alone other classes that are better at giant slaying.

Upgrade priority

The first upgrade you should always get, regardless of context or starting credits, is one tick of Explosive Headshot. After getting a single tick, it's generally preferable to max out your primary's reload speed, as this will allow you to shoot much faster and therefore trigger more explosive headshots. Next, make sure to get the remaining two ticks of Explosive Headshot to increase their range, damage and slowing ability. Once you've fully upgraded Explosive Headshot and reload speed, your upgrade path is generally open, and you can choose between upgrading your primary further and getting resistances if you're close to the front lines.

On your primary, you should get one tick of Health on Kill, and only one. Getting a multi-kill with an explosive headshot will more than likely completely restore your health. Following that, It's preferable to increase damage instead of charge rate, simply because it's generally advisable to repeatedly hit quickscopes than to charge up a full-damage shot. You may optionally take ammo capacity if you're not going to hover over a dispenser or your own private ammo box, and to this end you may also consider getting ammo refill canteens. Finally, special mention should be given to Projectile Penetration-- not only can it prevent a robot blocking you from headshotting a robot behind it, but if you hit two headshots with the same bullet, it will trigger two explosive headshots for double the damage.

Sniper is less reliant on resistances than other classes, but they shouldn't be completely neglected on later waves. Bullet resistance isn't a priority because Heavies will be too far away in most situations to do sizeable damage to you, and Sniper bots can be easily taken care of if you know where they are. Blast resistance is a good choice, on the other hand, because you will frequently draw the attention of far away Demoman and Soldier robots who will send many projectiles your way-- granted, you can dodge them, but it's still good to have some resistance to them just in case.

The only secondary worth upgrading is the Jarate, which can sometimes be very helpful in situations where your Scout's Fan O'War is either unavailable or impractical to rely on. Finally, the Bushwacka should have its swing speed upgraded if you plan on taking on the tank (just remember to bring the Cleaner's Carbine).

Tips for hitting headshots

Should go without saying that you should generally keep a fair distance away from your team to avoid taking any splash damage headed towards them, and to have a good vantage point to snipe from. Your most important targets are uber Medics connected to giants and tiny robots, but besides them, your priority should be the Heavies and Soldiers. They are the easiest tiny robots to hit, and have the biggest impact due to their larger health pools and devastating damage when attacking in a big group. Steel Gauntlets in particular deserve particular mention because they can be slowed to a crawl and cannot resist explosive headshot like they can for regular ranged attacks. Demomen and Pyros can occassionally be frustrating to hit, but are still relatively easy so long as you predict their movement and relax. Your Achilles' heel will be tiny Scouts, as they will scatter around and move very quickly, making it infuriating to aim for them. Don't feel bad if the odd robot or few slip you by, but just try to focus your aim towards the biggest and most condensed groups of robots.

Including the tiny Scouts, the most consistent way to hit headshots on small robots is to simply be on the same vertical level as them. This can be risky as you will often be in their path and attract their attention, and it's not always feasible depending on the map's layout. In most situations, you'll want to stay on the high ground and hope that the robots will be focused on shooting your teammates-- if they have their sights on you, you'll have to unscope after every shot or two to dodge the rockets coming your way. If you're not capable of quickly flicking your crosshair onto their heads, the most consistent way to hit your shots is to pay attention to what path they're taking and to line up your crosshair so that their heads come under it as they march forward.

When should you play Sniper?

A Sniper that's great at consistently hitting headshots can justify running him on just about any wave, but there are obviously certain waves that particularly favor Sniper. These would include:

Hamlet Hostility: Waves 1-5 (Waves 2 and 4 in particular are very easy for Sniper)

Bavarian Botbash: Waves 1-6 (Wave 6 should be avoided if your Soldier is doing all the work)

Desperation: Waves 1-3

Cataclysm: Waves 1-3

Mannslaughter: Waves 3-5

Broken Parts: Waves 1-6

Bone Shaker: Waves 1-7

Disintegration: Wave 3

You might be wondering what is it about these waves that makes them so great for Sniper, and what in particular creates good opportunities for playing him. Below are some situations where explosive headshots are particularly lethal, and waves where these situations are relevant.

The map in question is BigRock or Rottenburg. (Hamlet Hostility, Bavarian Botbash, Broken Parts, Bone Shaker)

Large amounts of small Heavies. (Cataclsym waves 2 and 7, Mannslaugher waves 4 and 5, Hamlet Hostility waves 2 and 4, Bavariant Botbash waves 1, 3, and 5)

Large amounts of Steel Gauntlets. (Hamlet Hostility wave 2, Desperation wave 3, Cataclysm wave 1, Mannslaughter wave 1, Disintegration wave 3, Metro Malice waves 2, 5 ,and 6)

Large amounts of small Soldiers. (Hamlet Hostility waves 1 and 5, Bavarian Botbash waves 2 and 4-6, Desperation wave 1, Cataclysm waves 1-3, Mannslaughter waves 3 and 4, Disintegration wave 3)

Large amounts of uber Medics.

BigRock and Rottenburg get special mention for a couple of reasons; Rottenburg gives you relatively easy opportunities to position yourself and to line up headshots, especially on the robots spawning at the cliff on the right side, and BigRock has all of the robots come out of the same small tunnel. The other reason is that both maps have what is termed a "godrock"-- more on this later.

Now that you know when Sniper is most convenient, you might ask the inverse question of when you shouldn't play Sniper. Some reasons would include:

Not being a good shot (duh).

You're on the later waves and everyone is skilled enough to spawncamp the robots, leaving you no time to headshot them.

The wave primarily consists of Scouts (such as wave 2 of Mannslaughter and wave 4 of Disintegration).

You're playing on Mannhattan, where it's awkward to keep an eye on both the pit and the catwalk, making Demoman a better choice (though Demo is preferred for much bigger reasons).

Godrocks

"Godrocks" are rocks which you may climb and remain invisible so long as you're standing on top of them. This might sound ridiculous to a new player, but it's very easy to make use of this trick; on BigRock, just climb the titular big rock from the corner between the rock and the fence behind it, and on Rottenburg you can climb it from behind. In this scenario, you will be invisible to enemies coming out of the tunnel on BigRock and enemies coming from the left spawn on Rottenburg, but you will become visible should they reach the exit tunnel on BigRock or the right side of the map on Rottenburg. To remedy this, there is an additional exploit that can make you fully invisible: Have the Engineer place a teleporter atop the small rock next to the small health pack and ammo pack on BigRock, and on top of the right edge of the right side rock on Rottenburg. This is only really feasible if the Engineer is queueing with you, but you can also take your chances and ask whoever's around if you're queueing by yourself.

Once you have a teleporter on the godrock, you can stay on top of it and never be attacked by robots. Provided there's a dispenser nearby, you can keep shooting forever!

Weapons

Primary

Weapon	Rating	Review
Hitman's Heatmaker	S+++	Half the reason why people play Sniper. By activating the focus meter, you don't unscope after firing and therefore don't need to reset your aim between each shot. Thus, you can hold down Mouse1 and treat the Heatmaker as a fully-automatic. The best option by a country mile.
Bazaar Bargain	B	A decent option if you want to specialize in slaying giants with fully charged headshots, despite this not being Sniper's strongest point. After maxing out Explosive Headshot and reload speed, you can max out your damage and not have to upgrade charge rate as you cannot increase the rate above 200%, which is the Bazaar Bargain's maximum.
Sydney Sleeper	C	Usable, but tends to be impractical. The main draw of this weapon is that the Jarate applied on hit also applies to explosive headshots, so you can theoretically coat multiple giants clumped together. However, you lose out on critical headshots, and it's usually much more practical to simply throw jars of Jarate instead.
Sniper Rifle / Machina	C+	Both of these weapons play out exactly the same way. They're not bad, but there's no particular advantage to using the stock or the Machina. Both of the Machina's upsides require you waiting for a full charge (a waste of time), and can be purchased as upgrades that apply to quickscopes. It's not the end of the world if you don't have any other options, but if you have the Heatmaker... just use it, trust me.
Huntsman	Bad	As already explained in the section about common mistakes, the Huntsman cannot trigger explosive headshots and is useless. No, spamming arrows at giants' heads is not a good idea. You're not funny.
Classic	Bad	Explosive headshots will only trigger on fully charged headshots, creating an unnecessary amount of delay between each shot that doesn't exist with the other sniper rifles.

Secondary

Weapon	Rating	Review
Jarate	S	There usually isn't any reason not to use Jarate. Applying mini-crits to giants or groups of robots you can't headshot is always helpful, and it synergizes perfectly with your Scout's Mad Milk. Upgrading the Jarate isn't generally necessary unless giant Heavies or Super Scouts are a significant threat.
Cleaner's Carbine	A*	The Cleaner's Carbine is mainly useful for applying full crits to the Bushwacka. Outside of that, there's no real reason to use it, but its synergy with the Bushwacka is great enough to warrant having it equipped, especially on tank waves.
Cozy Camper	B	Although made somewhat irrelevant due to the Health on Kill upgrade, having passive health regeneration can still be helpful for obvious reasons.
Darwin's Danger Shield	Bad	Not every wave has Pyros, and even on ones where they do spawn, you can circumvent the threat they pose by buying two ticks of fire resistance (very cheap!) or simply standing somewhere where they can't reach you.
Razorback	Bad	You might think that blocking a backstab attempt from a Spybot would be a great deal, but sadly they tend to hunt in packs. Even if you do block one stab, another Spybot will just kill you immediately after your shield breaks. Figure out a different way to avoid Spies.
SMG	Bad	It's not a big deal if you don't have any other secondary except stock, but the SMG brings no advantages in MvM. Even when it comes to self-defense at close range, you're better off picking them off with your melee or your upgraded primary.

Melee

Weapon	Rating	Review
Bushwacka	S	Synergizes perfectly with both the Jarate and the Cleaner's Carbine. With the Cleaner's Carbine you can smack a tank to death (make sure to upgrade swing speed!), and you can get some swings in on a Jarate-covered giant if you happen to be right next to it.
Any other melee	C	There is no distinct advantage given by any other melee. They're not bad if you need to defend yourself against a Spybot, but otherwise there's no reason to use anything that's not the Bushwacka.