I think this should be usefull enough as a newb guide and for some more advanced ones.
If needed I can do some map tactics too, but atm I´m a bit tired of typing.
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Demoman
or
How to be "Bloody brilliant"
The explosive expert of Team Fortress 2 is no class for "close distance fights", he mostly operates rather indirect, but he is still usefull as offensive and defensive force. Playing the Demoman the first time can be hard, because its a really complex class, due of the lack that he hasnt got any weapons which are able to shoot straightly at a target, so some players rather join a class like soldier or something else, but they who will continue playing it, for those are the following "How to"s, infos and tips.
Weapons
Primary:
The Grenade Launcher (splash)
Ammo loaded: 4 Ammo carried: 16
Damage: 65-125 Critical: 295
The grenade launcher shoots grenades which will bounce on any surface and will explode after 4 seconds or after hitting an enemy. You are not able to aim straightly at your target, so if you shoot the grenade will be influenced by gravity. At first it may be hard to hit the enemies at a longer distance but if you are more advanced you know which ancle you need to use. Also an advantage of the grenade launcher is delay from the explosion, because of that you are able to shoot at your enemies also they cannot see you, for example from above and also if they stand behind corners. Just make the gravity your friend. Sometimes it can be also advisable to shoot after prediction or just to spam them, believe me random frags are the nicest, because they come unexpected.
Secondary:
The Sticky grenade launcher (splash)
Ammo loaded: 8 Ammo carried: 24
Damage: 90-140 Critical: 336-353
The sticky grenade launcher shoots small bombs which stick to nearly any surface (except gates, enemies, buildings), they do not detonate, the player detonates them manually. Because of that they are very usefull to defend control points and to place "hidden traps", just place them behind a corner or at the ceiling, where the enemy cannot see them, and make them explode as soon as an enemy appears. The trajectory likewise will be influenced by the gravity and the ancl you shoot them, so you need to practice that too. If ubered Stickybomby are the best way for taking out Sentryguns, a good demoman can take out 2 sentrys at different places. Stickybombs can also be used to shoot over a great disstance, you only need to hold Mouse1 and a meter appears displaying the force you will shoot with. Another nice action you can do is similar to reocktjumping, shoot a sticky at the bottom, stand on it, jump and let the bomb explode at the same time and you will fly. Thats very usefull to reach places other classes cannot reach or to move faster. Warning! These tactic will hurt you 2 times, after the explosiond and after landing, so make sure you have got enough health.
Melee:
Bottle
Ammo loaded: / Ammo carried: /
Damage: 45-85 Critical: 195
The bottle is the only weapon for short distance and its also the only weapon which wont hurt you because of the splash damage. The bottle will break after hitting somebody or something with a critical. Only usefull as Anti-Spy weapon, if you dont want to waste ammonition or hurt yourself or if you wanna blame your enemy by hitting him from behind.
Playing the Demoman in general
Mapknowledge
Mapknowledge is the key to place your "stickytraps", you need to know from where the enemies will come and you also need to know, where you can place your sticky bomby, so the enemies wont see them.
Weaponhandling
Because of the gravitiy, the ancle you shooting with and the enemy movement, you need to know exactly how to shoot where so you will hit the enemy, the weaponhandling is the mainkey to become a good demoman.
Always fully loaded
Make sure both grenade launcher are always fully loaded, because the 4 grenades of your primary weapon are fastly gone and maybe you need to switch fastly to the secondary, and if this one is empty and you face a heavy, then my friend, you are dead.
Patience
Patience is also written big as Demoman, because sometimes you have to wait some time, untill some enemies will cross your way and your sticky bombs. Patience is the key for long kill streaks without dieing.
Sentry guns and ubers
Demomans are mainly used to destroy sentry guns, so if you will receive an uber charge, concentrate on the sentrys ahead, just dont care about the other enemys, after you placed your stickys retreat and enjoy the sound of crashing metal.
Sticky Jumps
Learn them, if possible call for a medic so he can buff you up to 150% of your health, after that there is nearly no place you cannot reach. At any map you will get an advantage by Stickyjumping.
Matchups
vs. Scout
The chance of killing a good scout is barely small. Because of his speed he can pass your stickybombs from one second to the other, if you dont look. And its very hard to hit them with the normal grenade launcher, you need to be very advanced to do this.
vs. Soldier
Easier to kill as the scout, but only in a "mid distance fight" because if you get to close you will hurt yourself too, and if you are too far away, he will not hit you, but the chance of hitting him is also ver small. Make his slow spped your advantage and use stickys.
vs. Pyro
At long range not such a big problem, also they are fast. At mid range advisable to place stickys between him and you, at a close range, if you are unprepared you got as good as no chance.
vs. Demoman
The best will win.
vs. Heavy
Try to catch a heavy when he is busy. Place stickys around him and make them explode, if he is still alive finish him off with your normal grenade launcher. If you face a heavy, who is alone, try to lead him into your stickys and avoid his fire.
vs. Engineer
Its unlikely to see an Engineer running around in openfloor, because the most time these creatures are hiding behind their sentry guns. These are one of your mainenemies, a good placed sentry is only hard to destroy, mostly only possible with an uber, a not so good placed sentry can be also taken out with your grenade launcher by "hida and shoot" or shoot out of a psoition where it cannot spot you.
vs. Medic
If you find one alone, just try and get him with your bombs. Its likely he will run or if he is brave, he will try and finish you with his bonesaw. Its up to you.
vs. Sniper
A good sniper will headshot you at a long range easily. You need to be fast and use every place to hide, stickyjumps are also usefull to get close very fast. If he is "zoomed" 2 or 3 stickys will kill him.
vs. Spy
Spys normally concentrate on backstabbing Medics, Heavies and Soldiers. The chance of getting a spy by hunting him with grenades is also very small, so just watch your back if you are waiting for enemies walking in yout sticky bombs and maybe try to "bottle" him or use some stickys.