The engineer. One of the most important classes in the game if used correctly, and a force to be reckoned with on defense AND offense.

The engineer is a low health class better suited to sitting at the back lines with his sentry than attacking at the front lines. However, in many situations a more aggressive playstyle is necessary. More about that later.

Base health: 125
(Buffed: 185)

Base speed: 100% (backwards: 90%)
Carrying a building: 75% (backwards: 68%) <- THIS IS IMPORTANT

You are slower while carrying a building. This means that it is easier to get out of danger as quick as possible (in most situations) than to stay and keep your sentry up alone, especially if the enemy team is right on your nest.

Your primaries are the façade of the building that is the engineer. They can sometimes be functional but most often don't affect your playstyle.

Shotgun ("Shotty"):
The most reliable close range weapon in the game sans melee. In its Scattergun form, it can be the most devastating as well. The shotgun is normally used in attacking or roaming situtations, but can be used in other situations with a bit of playstyle fiddling.

Basic info: 6 shots/clip, 32 shots carried

Frontier Justice:
Normally used in conjunction with the gunslinger, this is another attack-minded shotgun. The revenge crits mechanic can be used to W+M1 into the enemy team without worrying as much about the low clip, since it takes less shots to kill an enemy with crits than with none.

Basic info: 3 shots/clip, 32 shots carried

Rescue Ranger:
The most popular defensive weapon. Can be shot at any friendly building to heal it by 75 every shot. When used properly, the Ranger can be used to keep up a sentry where normally it would be rubble with only a wrench. Can be used in conjunction with the Wrangler to create the scariest engi combo short of the Gunslinger+Frontier Justice combo. Can also be used to pick up buildings long range at a cost of 130 metal and you are marked for death while carrying the building and for a few seconds afterwards.

Basic info: 4 shots/clip, 16 shots carried


Pomson 9000:
Can be used in pubs for some laughs but it's banned in any competitive setting for good reason. Interesting strategies can come out of using it, and it can be used in any situation in pubs with good effect, but it's best used in an offensive way (despite the depletion of spy cloak suggesting its use as a defensive weapon). Its depletion of ubercharge can ruin pushes, so use this weapon to flank the enemy and attack medics, especially if they're looking to push. If you don't have faith in your aim, simply find a safe spot behind the enemy and try to annoy anyone you can find with minis/constant Pomson fire, especially medics and spies. This weapon can ruin anybody's day pretty quickly.

Basic info: 4 shots/clip, infinite shots carried

Widowmaker:
Used in conjunction with the Gunslinger and usually pistol for extra damage in case you run out of metal. Useful if you have decent aim and can be used to decimate the enemy team if used correctly, especially with a Kritzkrieg (note: kritzing an engi is risky in all situations, but works best in a comp setting due to the high amount of communication). Great tool for improving shotty aim. Huge fun to use in any situation.

Basic info: uses metal for ammo - takes 30 metal per shot

Pistol:
Reliable medium/long range damage, good DPS (damage per second). Best used for harassment or when nothing else can be used. Works well on smaller maps with Gunslinger. Can be use in combination with Widowmaker for a consistent-damage-oriented playstyle.

Wrangler:
Probably the most used engineer secondary. Blocks 66% of all damage done to your sentry, doubles its fire rate, and allows you to fire rockets on demand (M2 while wrangling). This is best used while on defense, but can also be used with the gunslinger to harass enemies at longer range in attacking situations.

Short Circuit:
Useful in pubs, but will not be very useful in competitive. Main uses in comp include destroying stickies on spawn doors when attacking on a payload map and the rare use as a spam negator on defense.

Wrench:
Reliable, balanced, fun. Solid choice for someone that has incredible team support and doesn't have to bother with the enemy team at all.

Southern Hospitality:
My personal choice. Lets me deal with spies easier so that our team's pyro can roam more. The downside is effectively negated in comp considering that most pyros will not W+M1 into a sentry at that level, and the chance of being caught off guard by a pyro is almost zero due to communication and protection of the flank.

Jag:
Useful for someone whose sentries go down more often then they would like. Lets you build up quicker, but requires a pyro to stay around you to deter spies because of the reduced damage. Useful in comp, but not as much in pubs considering that more classes will flank you due to lack of comms and lack of flank protection.

Eureka Effect:
Very interesting playstyles can come out of this wrench. I've never used it in a competitive setting, but I've never liked it very much. It sacrifices fast building for the ability to teleport to spawn or a tele exit, the former of which is relatively useless and the latter of which is only useful if you're rolling and want to try a gimmicky sentry flank.

Gunslinger:
most op wep in game pls ban Kappa
What a fun wep. The triple punch crit mechanic is useful for taking
out clueless heavies (and can be scripted to allow crits on demand). The minisentry is incredibly useful on koth, cp, and attacking on payload due to it's fast setup and, in the case of koth and cp, area denial to keep enemies off of the point. Makes people angry. Don't use this with the rescue ranger, you won't be able to do much.

The more aggressive engi playstyle comes in hot on these gamemodes, and can be maximized with situational loadouts.

Loadout #1
Shotgun
Wrangler
Gunslinger

Comp:
Use this loadout on larger KOTH maps and on PL Attack. It lets you be aggressive with the shotty while spamming out enemy positions with the wrangler. The minisentries you put up can win the match for you, especially when the sentry spots are:
-hard to see
-not spammable
-able to cover the point well and prevent peeking

Lakeside is a good example of a map where this loadout is king.

Pubs:
Use this loadout to spam people out and to snipe people from across the map. Putting minis in sneaky spots will give you lots of kills because of the lack of communication between players. They just won't know where it is. You can still be aggro with this loadout, but if your shotty aim isn't spot-on, you won't get as far.

Loadout #2
Shotgun/Frontier Justice
Pistol
Gunslinger

Comp:
Useful on smaller KOTH maps and on CP maps, like Viaduct and Granary. Used to make you another flank player that also has minis on the point, which gives your team a huge edge on the flank, which can win games in many situations.

Pubs:
Aggro time! Maximum damage means maximum W+M1, so get in there, plop down a supportive mini, and wreak havoc.

The basic playstyle of the aggressive engineer has a few tips to go with it, regardless of loadout or place of play (pub/lobby/match/etc):

1. ALWAYS have a supporting mini with you when attacking - this will win fights

2. ALWAYS seperate yourself from the mini when fighting 1v1. This does 2 things for you:
a) makes it so that you won't be damaged by the mini trying to shoot through you
b) makes the enemy choose between you and the mini (hint: they normally choose the mini, giving you more time to attack

3. ALWAYS build up any supporting buildings before geting aggressive, engi is still a support class by heart no matter what playstyle you use

4. ONLY enter fights you know you will win. This also applies to 6s scout and I think it fits well in both situations. It's especially relevant for engineer in that you are more important than other flank classes and it's a good idea to die much less than them so that you can keep supporting your team through dispenser/teleporter building and upgrading

THE MOST IMPORTANT PART OF THIS GUIDE. PERIOD.

Comp:

This section isn't as much about loadouts as it is about strategy. The definitive competitive loadout for defense is:
Rescue Ranger
Wrangler
Southern Hospitality/Jag

That's all you need to know. Wrench depends on how your pyro likes to play (roaming pyro = hospitality, pybro = jag).

This makes your sentry a TANK.

As seen in the Barnblitz clip at 2:09 in the video below, using this combo can save a sentry from an uber right in your face, but it requires practice, timing, and lots of luck.


UBER PUSH CHECKLIST:
1. Is your medic alive?
2. Does he have uber?
Yes: you're probably good. Have your team push their uber out with their own.
No: rely on your pyro to keep them out.
3. Is your pyro alive?
Yes: rely on your pyro to keep them out.
No: prepare for them to push into your sentry as soon as they can.

AS THEY APPROACH THE SENTRY:
1. DO NOT let yourself into their line of sight. They will decimate you and then go for your defenseless sentry
2. DO wrangle your sentry ASAP
3. DO NOT worry about having a line of sight to the enemy combo. All you need is the shield on your sentry.
4. DO try to knock them away with rockets if you have a safe line of sight - especially if you can knock them off a cliff (upward, steel, even badwater roof)
5. RR (rescue ranger) as long as you can while the shield is up, then bring up wrangler after if starts fading, then repeat. If you are in a safe location with a dispenser (barnblitz last is a good example) use your wrench instead after one clip of RR healing to make sure that you don't have to reload at a crucial moment. You should be able to ward off a demo easily, and a heavy right up on your sentry with luck. THIS IS THE BREAD AND BUTTER DEFENSIVE STRATEGY.
6. IF a demo spams your sentry with stickies BUT DOESN'T DET THEM you are in danger of losing your sentry when he does. Simply wrangle some rockets at the ground next to your sentry and push them away.
7. TAKE CAUTION when wrangling rockets while standing close to your sentry, you might accidentally pick up your sentry, which will be a disaster. For this reason, only spam rockets if you need to or if you are further away from the sentry
8. DO NOT fire at your enemy with the wrangler UNLESS you are sure you can move them to a non-threatening location (keep in mind that only the pocket can be moved by a sentry, medics are immune to sentry knockback while ubered)

These strategies are also applicable in pubs, but you won't have as much team support.

DEALING WITH SPIES:
If using Jag, hit sapper once then attack the spy with everything while calling for support (applicable in comp only, in pubs comms are nonexistent and don't get listened to if they are being used). If the spy backs off, then hit your sentry again and let it take care of the spy that now is too far away to sap again.

If using southern hospitality, hit the spy once then the sapper twice, the bleeding should force the spy to back off a bit. In a comp match, the spy might be more determined and will probably not care about the bleeding. If he saps it again, hit the sapper once and use the Jag strat.

ROLLOUTS:
1. Tele entrance down at spawn
2. Run back and get metal from the supply cabinet
3. Use suicides* to build up your buildings in this order:
-Sentry (to level 3)
-Dispenser (to level 3)
-Teleporter (level 1 is all you need, but if you have more time go ahead and upgrade it more)

*only pick up suicides if you have 100 metal or less (every weapon on the ground is 100 metal), that way you can maximize your metal without wasting any

SENTRY PLACEMENT:
coming soon w/ videos :3

Pubs: Everything above but be more conservative. Don't hope to keep your sentry up in most situations, just back up and grab your sentry with the RR (right click while mousing over the sentry).
Also, never built your sentry next to another sentry. Just don't. Even if the other person is asking for it. Just don't.

FAQ:

Q: Why isn't the Widowmaker in the loadouts for aggressive engineer? You said it was viable!
A: It's viable for players with great aim, and even then requires great coordination and positioning. It is also only really useful in specialized situations that don't happen often.

Q: How do I sentry jump?
A: Sentry jumping is useful, but not necessary. It is nice to know, however, so here we go.

Level 1/Level 2/Mini: Stand on top of your sentry with the wrangler, point it up, and wiggle so that you see the laser coming out of the top of your head. Now fire your wrangler while crouch jumping for as long as you need.

Level 3: Stand next to your sentry, point wrangler at your feet, and crouch jump right before pressing M2 (which fires rockets). Surf to your destination.

Q: Why do I only hit the sapper once when fighting a spy with the Jag method?
A: It prevents the spy from getting another sapper down while you fight him and makes it easier to destroy when you've killed the spy.

Q: Why shouldn't I build my sentry next to yours? Doesn't the nest do more damage that way?
A: Because it makes it easier for the enemy to destroy the nest. Having the sentries right next to each other just makes it easy for a demo to kill two birds with one stone (or two sentries with one sticky in this case)

Conclusion:

Thanks for reading!
-Post in the comments if you have any suggestions for the guide and if you have any short questions
-Add me if you need an answer to a complicated question

Resources:

http://twitch.tv/sigafoo - sigafoo's (platinum engi main) stream
http://comp.tf/wiki/Engineer_(Highlander) - comp.tf page for engineer
http://gtm.steamproxy.vip/groups/engimains - my engi main appreciation group
http://playcomp.tf - how to get into comp TF2
http://twitch.tv/plextf - my stream
http://www.youtube.com/watch?v=Yk-6NXCo5ZA - the engineer bible (great tutorial for many engi strats but somewhat out of date)
https://www.youtube.com/user/sigafoogaming - sigafoo's youtube channel (demo reviews and clips)
http://forums.tf2center.com/topic/775-playing-engineer-in-highlander/ - guide to competitive HL engi by MightyBurger

Happy engineering!
plex