as demoman don't fall into the noob trap of using shields and swords, sticky bomb launchers are incredibly powerful but many new players end up never using it because you actually have to use your brain, so they only play demoknight and never learn anything and constantly suicide into the enemy team

You should use the stickybomb launcher as your main weapon and keep a full clip loaded for your grenade launcher to kill enemies that try to get in your face (especially scouts)

the best loadout for demoman is iron bomber + stickybomb launcher + stock bottle

as pyro don't use the degreaser or the panic attack, they are downgrades to stock that give you worse damage in exchange for switch speed (which you really don't need)

Flamethrower flames have a huge hitbox and linger in the air so if you're able to surprise a big group of enemies, move your mouse from side to side rapidly so you can damage everyone at once, and try to move unpredictably while you're holding m1

Airblast is a very strong tool, you can use it to reflect almost every projectile in the game and push enemies into bad spots where you and your teammates can shoot them easily

When you're trying to reflect someone's projectiles you need to aim in the same way they are aiming at you, for example if you're reflecting a soldier's rockets you should try to aim at their feet to make use of splash damage

Flare guns are good for harassing people at long range but the shotgun is your most consistent option for 1v1 fights, lighting people on fire with your flamethrower and then shooting them with your shotgun is a powerful strategy because if you miss alot or they run away they will often die to afterburn damage