Engie is currently my most played class, so here are a few things I figured out along the way.
- On most maps, you should set up Teleporters first. Your team will love you.
- If you want your team to hang around your location, build a Dispenser where it will be useful to them.
- Often your structures will take longer to build than you have time for, so don't feel obligated to wack every thing you make to completion. Often your presence will call attention to your intelligently placed sentry and its cover will be blown.
- Don't care about points, because you'll be handing the enemy team a ton when you are dropping lots of sentries, which if you are being aggressive may turn out to be quite a few!
- In general, don't rely on your team to watch your back. Most of the time you'll be disappointed if you over-invest in a location.
- Be Persistent! Like Kayin mentioned earlier, even if you can't rely on your team to protect your equipment, often times your opponents team won't self-organize themselves in such a way to take maximum advantage of the hole a skilled Soldier/Demo just opened up.
- Don't take my stance to mean that you should be bullheadedly stubborn, because if your stuff is being blasted ALL the time it's likely that you're not building it in good spots. Be persistent, but be flexible and creative, too.
- Don't be afraid to play engie on stages where the battles are very mobile, but also don't rely on it as a stable strategy. Some of my best engineer moments were on maps like Fastlane. Most of the time just build a surprise sentry or two, and then switch classes.
- Be mobile. Like I said earlier, as long as you keep your feet moving you'll find that metal is exceedingly plentiful. Keep building sentries on odd locations that your opponents won't be expecting. Often you'll get kills just on account that they don't know where the damage is coming from. A engie with his shotty out and a sentry adding to his damage can bring a lot of pain.
- Along that line of thought, don't be afraid to mix it up in direct combat. Due to the way your sentris damage applies to your crit%, you'll find that you do more crits on average than other classes, especially in melee. An Engineer that doesn't afraid of anything can defeat some unexpected opponents.
- All building setups are situational. Sometimes you want the dispenser within arms reach so that it can supply you with health and ammo while you tank your sentry. Most of the time, doing that is a waste of time and resources. Shadow Moses has a larger-than-average concentration of skilled Soldiers and Demos. As explosions happen to be anathema to your equipment, investing a lot into closely-arranged stuff is folly.
- A decent set up if you MUST have the security of a nearby dispenser is built your sentry at the farthest possible distance from the dispenser, where you can just barely wack the sentry with your wrench, and also just barely be in reach of your dispenser. Build the Teleporter exit a small distance further, so your team can get to your posistion and help defend it.
- And, most importantly, play drunk. Or at least in a state of mind where frustration won't set in. Honestly, if you are getting frustrated no amount of advice will aid you. Same goes for any class, but Engie is particularly susceptible, given their reliance on a great many factors that are out of your hands, such as team composition and a fair amount of luck. If all else fails, dance. Others will join you and your engie rush will be glorious indeed.