What are effective roles for each class in Mann vs Machine?

Solution 1:

UPDATED for Two Cities!

In short:

Scouts:

• Pick up money. They have a huge pickup radius. No money means no upgrades and you can't win without those. Money picked up by scouts grants overheal with no cap.
• PICK UP THE MONEY
• Throw milk to slow groups (with upgrades).
• Use the Fan o` war or upgraded sandman to mark giants.
• Use the Force a Nature to push bomb carriers into pits or reset it entirely!
• Attack single medics who have already popped uber
• Eliminate pesky snipers, spies, etc. in hard to reach areas.
• Distract giants into shooting (and missing) you and not your teammates

Soldier

• Damage waves of enemies and attack the tank. Kritz is useful here.
• Stun Giants, especially scouts and medics, with the rocket specialist upgrade
• Can rocket jump to move quickly and reach pesky enemies (snipers, spies, engineers, scouts with the bomb).
• Buff Banner to give everyone mini-crits.
• Good at attacking single targets (giants, tanks, lone bomb carriers) due to maneuverability.

Pyro

• Damage waves of enemies. Kritz is very useful here.
• Attack the tank with the Phlog. Ammo refills are very useful here
• Specifically ignite enemies and run away, letting afterburn do a lot of damage to enemies (especially fast scouts).
• Airblast the bomb carrier and juggle ubers. Airblasts can buy more time if the bomb carrier is on the hatch. Don't expect to reflect projectiles though.
• Airblast the bomb carrier off of bridges or into holes, either delaying the bomb or forcing a reset. NOTE not all maps have holes/hatches for resetting.
• Generally as front line attacker, they can also pick up money.
• Spy checks.

Demoman

• Set sticky traps. Combine this with a before round kritz to lay effective traps for tanks, groups of medics, giants, etc. You'll use your sticky launcher more than your pipe launcher.
• Damages large groups of enemies and attack the tank. Kritz is very useful here
• Sticky jump to move around and clear out snipers, spies, bomb carriers, etc.

Heavy

• Your main attacker. The heavy shoots down waves of enemies and damages the tank and giants. Give him kritz and ammo.
• Give your sandvich to a teammate.
• Use knockback rage to delay forward progression of the bomb carrier
• Stand near corners/tight areas and body block giants, especially super scouts.

Engineer

• Set up Sentry Guns. Sentries, like the heavy, are your main damage dealing to enemies. Keep it alive. Watch for sentry busters!
• Set up Dispensers. Generally left by the Heavy and the Engineer picks up their own metal. It also heals hurt teammates near the front lines.
• Set up Teleporters. After death, teleporters save valuable time that would be wasted walking.
• Use the wrangler to shoot down snipers from a distance safely.
• Use the short circuit to destroy projectiles from enemy soldiers and demos.
• Can setup the sentry gun near a corner/tight areas to body block giants, especially super scouts.
• Can you your sentry gun to force a sentry buster to explode at a spot near lots of robots

Medic

• Heal and keeps the team alive. Duh..
• Use the shield to protect teammaters from incoming damage. Very useful, use this as much as possible. Can be used to clear a wave of regular robots or spies from a crowd.
• Kritz/Uber (from medigun or shared canteens upgrade) to mow down the enemy. Kritz the heavy or pyro for tanks. Kritz a demo before the round to lay crit sticky traps.
• Revive fallen teammates. When players die they drop a box on the ground with a hologram of their class. Medics can "heal" that box and revive players.

Sniper

• Headshot giants for extra damage. Combine with explosive headshots to clear groups of robots.
• Throw Jarate to give more damage to giants or waves of enemies. Will slow if the upgrade is chosen for it.
• Sydney Sleeper gives jarate effect to giants and other enemies.
• Counter snipe other snipers.

Spy

• Sap waves of enemies. The sap also reduces giant's armor and speed. This is very important for waves with giants and uber medics.
• Stab Giants. In later waves, with many upgrades, you can stab giants for high damage before they turn and shoot you. You can solo giant medics as they won't turn around.
• Stab groups of enemies, especially sapped ones or medics.
• Destroy robot engineer nests.
• Pick up money. As a front line attacker with a disguise the spy can easily pick up money.
• Go after pesky snipers.

A general setup for the Two Cities maps is engineer, heavy, scout, medic, demo and soldier. This is flexible but the most common and successful setup I've seen. Generally Snipers aren't favored. Map selection and current wave lineup can favor or handicap other classes like pyro, spy, sniper, etc. Generally a scout switches to a different class on the last wave and refunds their upgrades for whatever is needed (phlog pyro for tanks, spy for giants, second heavy, etc.)

This is all from my general experience in playing. There is more I'm sure I'm missing which I'll add later. Keep in mind everyone plays the classes a little different: Some scouts like to use Bonk, some soldiers like to use the concherer, etc.

Solution 2:

The balance of classes is certainly skewed a bit in MvM, but I wouldn't say that any class is useless. Certainly some classes are more useful than others though.

• Strictly speaking, an Engineer is almost required outside the easy waves. Aside from the obvious using Sentries to thin the herd, upgraded players will probably be burning through ammo and need Dispensers to refill. Once their base is set up, Engineers should be divided between pestering robots with distant shotgun/pistol fire, restocking his Sentry, and dodging constant Sentry Busters.
• With a good Dispenser spot, a Heavy gains basically infinite ammo and can bullet spam into the crowd. Only once a giant shows up is it likely for him to need to retreat.
• Soldiers and Demomen should be putting as many explosives into the horde as possible, and like Heavies should be getting most of the manual work done.
• In most cases you need a Scout to get the money, or you either won't have enough upgrades to win or die trying to pick it up as another class that wasn't already fighting at close range. Scoring mad cash is basically the Scout's main job, but they should also be smacking things with milk, baseballs, or war fans when not putting steelshots into things.
• It's not hard to win without a Medic, but the money spent on Über and Kritz canteens can add up. Medics should generally always be running Kritz and charging Heavies, Soldiers, and Demos; though Über isn't a bad idea when it comes to protection from huge/dangerous waves, it's generally more important to blow things up quickly and the extra charge time can be a problem. Medics should also Kritz Demomen during setup to lay crit sticky traps, especially for Tanks. Otherwise, a Medic should simply try to stay out of harm's way and heal as many people as possible.
• Spy is highly useful when played right - sap a crowd, backstab whoever got away, use the Dead Ringer when in trouble (robots are oblivious to decloak noise), and sap'n'stab giants until they spot you. Of course just like regular play an inefficient Spy is not much more useful than a dead one. Spies can also help get money if the Scout is away but he has better things to do.
• Sniper is not a bad class once he starts getting upgraded, as piercing shots and faster charge can quickly down a big mob or a giant robot that can't fight back at range. Jarate is also a safe source of mini-crits to the team. But like the Spy, a Sniper needs to have enough skill (and computer power) (or arrows) to not be dead weight until then.
• Pyro has problems in MvM; like the Scout he has to get inside to do anything, but unlike the Scout he doesn't have ludicrous overheal from cash pickup or high mobility. Still, the airblast is indispensible for backing up the bomb or dropping it into a pit and can put out burning allies, and once the flamethrower or the Pyro's resistances get upgraded he can do some pretty good work on the enemy swarm. Being in close range also makes the Pyro an okay cash-grabber. The most supporting class of the mode in my opinion.