The Demoman is a great class for Mann vs Machine as he has both unique offensive and defensive capabilities in the grenade launcher and the sticky bomb launcher. The grenade launcher has is best use in the early stages of each round. Because it’s both an explosive weapon and has a good rate of fire, it has the potential to do a lot of dame to a large number of enemies really quickly. When your on the front lines you’ll want to scope out around you to find out where the ammo and health is as the grenade launcher runs out of ammo really quickly. If you have an engineer on your team, being near a dispenser is great. Though don’t get to attached to it as Engineers always place them near their turrets, which get taken out by Sentry Busters pretty often.
As far as upgrading the grenade launcher there is a few tactics to go with. Upgrading ammo capacity is good and will allow you to wander the map with some more freedom without the fear of running out. Same with upgrading the clip size, but you’ll want to upgrade the reload speed with it side by side. I also like upgrading the projectile speed, as you won’t need to lead your target as much and you’ll be able to aim better. Don’t go over board upgrading your grenade launcher though. Unlike some of the other classes, the Demoman needs to be upgraded in a more well rounded manner to be fully effective.
Same as when playing as the Soldier, a good tactic is to get to a higher vantage point. This way you can keep an eye on the enemies and the bomb once they start to break away from the main group of robots as you’ll want to move to a better defensive position once this happens. This is when you’ll want to use you sticky bomb launcher. Dropping a couple sticky bombs on either the bomb or the defense site is a good emergency measure against the robots. The difficulty with this tactic is replacing the bombs after you set them off. For this reason I like to upgrade the firing speed on the sticky bomb launcher. This way you’ll be able to continuously blow up every scout robot that tries to pick up or set off the bomb.
As for playing Demoknight in MvM, it has a paradoxical way of functioning. Demoknight doesn’t really play well in Mann vs Machine, but it seems that MvM is a good place to learn how play Demoknight. A basic strategy tips are to loop around the wave of enemies and charge through them from behind. My favorite upgrade method for Demoknight is to just add regenerative health and regaining health from kills with your sword. This way you’ll stand a chance of surviving for a long time in the center of the chaos.
Overall Demoman is a great multipurpose class for someone trying to be a good assist character, but still want to see some action and get some kills.
Article from Gamersyndrome.com