The bread and butter general assault unit, the Soldier is one of the most common and simple-to-understand classes, but that doesn't make it one of the weakest. Infact, when played right the soldier can be one of the most dangerous classes in the field. Equipped with a rocket launcher as their primary draw and a large health pool without being terribly slow, the soldier is a great class for many offensive and defensive roles and easy to pick up and play. But with the slow stray of the rockets and the fact that he's got them only four at a time makes the soldier difficult to master. Anticipating the enemy's movements and knowing when to shoot is important.
Rocket jumping is one interesting technique they're known for. Aiming your Rocket launcher toward the ground and shooting/jumping simultaneously will cause the Soldier to bound to incredible heights, at the cost of a bit of health. Once mastered, this skill allows Soldiers to pop up in unexpected places and assault enemies before they knew what hit 'em.
Arguably, the Soldier is also the best Medic partner when a Heavy isn't around.
* Try Rocket jumping to the tops of structures that no other class would be able to reach. You'd be surprised how often even a guy with a big Rocket launcher can surprise the enemy when they're not where people are expected to be in a map. Note that you can also take fall damage upon landing.
* Always keep your Rocket launcher reloaded when you're not in the middle of a firefight. With only four shots of your primary weapon between reloads for such an assault-built class, it's best to always be prepared.
* Rockets travel slowly, so always aim at the feet of the enemy or aim at a wall. Although a rocket has a much greater point of impact damage than the splash, shooting enemies directly is too risky and you'll probably just waste your precious and all too few ( only 4 see?) ammo. You can try shooting enemies directly only when you know that you get the shot or in tight corners when you gotta do a LOT of damage quickly. Also, a well-placed foot shot will throw (juggle) the enemy, slightly disorienting them and giving you a more predictable place for a lethal follow up shot.
* When shooting over long distances, try to lead your shots and shoot a second rocket where they're likely to retreat to. This will greatly increase your chance of actually hitting them over long range. If the enemies are too far away you can just tease them a little bit with your shotgun too, rather than wasting your rockets. This wont do much damage but the shotgun can be loaded much more quickly and the bullets hit the target right away. And it's damn annoying to be under fire all the time.
* Rockets reload slower than most weapons. It's unlikely that all four will hit if fired consecutively. Take your time in predicting your enemy's movements and fire one rocket at a time towards your enemy's predicted destination.
* If you do empty your four rockets in a fight and there is an enemy nearby, consider switching quickly to your (fully-loaded) Shotgun rather than waiting for the rockets to reload: this will allow you to continue your attack uninterrupted. Even with just your secondary weapon, as a Soldier you are more than a match for any enemy in range. Also, if you're stuck in a duel between two Soldiers, switching to the Shotgun could be the difference between killing your enemy, or him killing you.
* Help your Medics out in areas where there is no fighting by standing next to a wall and shooting it repeatedly. Damaging yourself allows them to fill their ÃœberCharge faster. You can charge more than 1 Medic at a time. Note that ÃœberCharge builds up faster during setup time on the PC version, so this is only useful after setup.