Players can purchase upgrades for their weapons using in-game currency, called credits. All purchased upgrades will last for the duration of the game.
If a player leaves during the game for any reason, providing they reconnect, their upgrades will be retained for 3 minutes. Also, their spot on the server will be reserved for that time period. Between waves, no robots will spawn whatsoever.
MvM's backstory is explained in the comics "Blood Brothers" and "A Fate Worse Than Chess".
In the former, Redmond and Blutarch Mann come to a truce, only to discover about their long-lost brother Gray, who kills them shortly after. In the latter, Saxton informs RED and BLU that they're out of a job, and re-hires them as a united force to combat Gray's robot army, beginning Mann Vs. Machine.
The boston maniacs job in MVM is to collect the money dropped by robots. Your speed will allow you to run around and pick up all the cash that your damage dealing team members cant get to. The best upgrades are resistances so you will be less susceptible to being killed when your bobbing and weaving through the robots trying not be targeted picking up loose change. Good weapons to support your team include the Sandman to stun bomb carrying robots, Mad Milk to help your team get health, and Bonk to distract big robots and give you an edge when picking up money. You dont want to be focused on damage output when playing Scout but don't shy away from shooting robots and picking off medics and snipers.
Good mvm maps for scouts: Mannhattan, Decoy, Coal town, Rottenburg and mannworks
The soldier has huge damage output and to help your team the most you should find a high spot with a blast jump and spam the robots below. Banners help a ton for you and your teams damage output against tanks and robots as well as the Conch helping them stay alive. huge oppertunities for team play.
Good mvm maps for solider: all maps as solider is a all-around class
The pyro needs to be upclose and personal with the robots at all time so I strongly recommend uber canteens when it gets too hot, get it. He can airblast the bomb holder back and help the team a ton as well as doing absurd damage to tanks. The manmelter is a great option when faced with robo pyros as you can extinguish your teammates and get some CRITS. The flare gun may seem like a good idea but you should stick to your flamethrower most of the time.
Good mvm maps for pyro: any map with environment hazards (eg.rottenburg, coal town)
Good mvm maps for demoman: any maps with two main robot entrance (eg.mannworks)
Heavies will be the main source of damage so a medic might heal you and revive you more than anyone else. Note Natascha will slow enemies and longer the time,then more time is needed to deploy the bomb.
Good mvm maps for heavy: any
Engineer you are the second best class in MvM so please build a sentry and dispenser, Rescue Ranger and building level-up canteen will help your team win easier. Pro tip: use 1 canteen for multiple buildings .
Good mvm maps for engineer: any
Medic you are the best healer in the world hat can save your teammates in this gamemode! Bringing the Kritzkrieg will increase damage output (this gamemode is for mass damage output, not mass-healing! Medic can switch between stock Medigun and crit canteens or kritzkrieg with uber canteen.) for your team.
Good mvm maps for medic: any
Sniper (not guaranteed as you might be vote kicked) get bullet piercing and bring Sydney Sleeper will also increase damage output, remember, target medics first, then giants, and the rest are optional.
Good mvm maps for sniper: a small spawn for robots(eg. Mannhattan, bigrock, decoy)
Spy, your main objective is to sap ubercharge medics or waste ubercharge, so Ambassador and Cloak and Dagger will protect you from death or anything else. Using ammo refill canteen can instantly refill cloak bar and Sapper.
Good mvm maps for spy: bigrock, Mannhattan.