(Note: The Frontier Justice will be referred to as the FJ)


What the game doesn't tell you:

• Has bullet spread, which means the gun can hit multiple people
  
• Min-Max damage possible w/ FJ: 18-180 (On one person.)
  
• Can only store up to 35 crits, the size of your max clip and ammo combined (32 + 3)
  
• For every kill made only by the sentry (or with you) alone gives you two crits instead of one when the sentry is destroyed.
  
• For every kill assist made by the sentry you get one crit
  
• You can only deal multiples of 18 in damage up to x10 while in crit mode.

Kills Vs. kill assists can be recorded by the sentry HUD. For every kill made just by the sentry, the counter on the left of the hud will go up. For every kill your sentry assisted in, the counter in parenthesis will go up. (Keep in mind that if you kill someone alongside your sentry, you will not cause a kill assist.)
This is explained by this crappy image made in paint.net!

Considering the fact that you have half the clip of a normal shotgun when using the FJ, it goes without saying that you need to aim well. A good place to practice aiming is on 24/7 ctf_2fort. The reason I recommend 2fort is because of the tight corridors of the map, which means that there are lots of chokepoints, where many enemies may tunnel into at once, and that there aren't many places for other classes like Scouts and Spies to go in order to assault/run away from you. Good places to practice are the sewers and the intel hallways. Nontheless, always remember that you have half the clip of the shotgun, and that missing all three shots means that you need to reload or swap to a secondary/melee.

and/or that you just wasted three crits, idiot.



Another thing to remember is the amount of damage the FJ can do when you have crits. As stated before, the minimum and maximum amount of damage you can do to someone with FJ crits are 18 and 180 respectively. That means that you can take out a fully overhealed heavy with 2.5 meatshots!

Don't get too cocky though, because you have to be right next to someone to have a chance of landing a meatshot on them. The most common amount you'll do to someone on a meatshot is 108. Not to say that isn't an impressive amount.

These loadouts benefit the FJ in one way or another. Basic playstyles will be explained in these loadouts. In other sections, the names of these loadouts may be used. I will also be listing the loadouts on how good/un-situational I think they are.
Thirsty for Crits
FJ
Wrangler
Any

Thirsty for Crits is a loadout I thought up while making this guide. It is probably the most wasteful you can be with the FJ, because of the fact that you don't have a weapon secondary like the pistol or even the Short Circut. Why would you want to use this loadout? The only asset I can think of is that you're using The Wrangler. If you have any idea why it would be good, let me know in the comments.
The Mother Hen / Stockpiler:
FJ
Short Circut
Any (except gunslinger, because the SC would be near meaningless then.)

I call this loadout the mother hen because you're protecting the sentry, and when the sentry is destroyed, you get the crits from the sentry (applied that it actually managed to kill someone.) This loadout is good for anyone who wants to stockpile crits. It is not recommended to use the gunslinger because Mini sentries are made to be disposable, making the SC impractical.

also because you don't want to take the Mother Hen to another level

On the Offensive
FJ
Pistol
The Gunslinger or The Jag

I really can't decide which is better because both melees are viable for this loadout. You could argue the Southern Hospitality, but this loadout is made to benefit the sentry/buildings.

On the Offensive: Gunslinger

The Gunslinger is in the loadout because of the +25 Hp boost it gives you and the mini sentry. This kind of playstyle is drastically different from The Jag because of the mini sentry alone.
The playstyle of the Gunslinger Offensive involves laying down the sentry, getting a kill/kill assist with it, having the sentry be destroyed (By enemy or manually.) getting crits, repeat. The pistol is in both loadouts because it allows you to attack without having to give up your crits.

plus dat 200+ ammo doh

On the Offensive: Jag

The Jag is a more Stockpile playstyle. You would create a nest in a chokepoint (e.g. The sewers in 2fort) Get about 4 self-kills, destroy it on purpose (usually) and build the sentry back up to level 3. This loadout is useful because you can surprise Medic-"" pairs who just finished with their uber and laying waste to your nest, thinking that the battle only involves finishing you off. However, those un-critted critters don't know is that you've been waiting for prey like them to shoot the living hell out of!

https://wiki.teamfortress.com/w/images/1/12/Engineer_taunts08.wav?t=20100709023109

Fighting a Frontier Justice Engineer is rare. Being a Frontier Justice Engineer is cool. Now listen up to make sure people don't decimate your parade with bullets, rockets, grenades, hats, cabers, sappers, knifes, Baboon hearts, fire, compressed air, trades, scrap, keys, and hats.

and get in fights

I will only be covering the Frontier Justice in these matchups unless there's a suggested alternate way of murdering someone. I will also be covering other major playstyles. And another thing; I'll be expecting that you have crits most of the time.

The Scout:
As said in the Engineering Bible, a good Scout will wreck you. Not even crits will really save you from the Scout. I can't tell you how many times I had several crits stored up and some scattergun Scout body shot all the electrical neon red/blue color out of me. Using your sentry against the scout is a much more viable option, since that clip in meet the scout isn't true. You know the one...

Anyways, if your sentry is not with you, try running the scout to your base or into a chokepoint where your team members are abundant.






The Soldier:
The Soldier, as once again said in the EB, will either be dumb cannon fodder or your worst nightmare. I'll skip over this part mostly as you can just go into the table of contents in the EB and watch that section, but the FJ wasn't covered, so I'll go over that here.
The FJ is very effective against soldiers close range. A full crit body shot and a few shots from your pistol will lay waste to that 200 hp. If the soldier is being overhealed. Two full crit body shots will still do the trick, but it's still likely you would be killed in the process. Bobbing and weaving will make sure that you only take splash damage and that Direct Hit Soldiers will have a hard time. Also, DH Soldiers will bust your sentry. I suggest just letting it happen if your sentry has kills, so you can get the drop on them, putting Soldiers into a panic state.


The Pyro:
Trying to use the Frontier Justice against Pyros is a difficult bout for both. The Pyro knows that one good body shot from a crit FJ will kill them while the Engineer knows that if he gets too close in an attempt to score more damage, he'll likely catch on fire. It's recommended to stay mid-range from the Pyro and just use the pistol if possible. Keep in mind that you're more than likely the same speed (exceptions include The Powerjack) so running in a straight line backward away from him isn't the best idea.

I say this when I said that a good place to practice is the sewers of 2fort


The Demoman:
The Demoman is seen to be the highest DPS class in the game. Really, you shouldn't mess around with the Demoman. 175 Hp is enough for a full body shot to kill him, but his explosives usually keep you away with their knockback. Unless he's using the Scottish Resistance or Sticky Jumper, don't try saving your sentry if the stickies are under/around it or run into stickies immediately after they were launched. That being said, you should still give him a taste of charged bullet. Speaking of charged...


The Demoknight:
Dodge his charge if you can, then do a crit hit. You're really the better class if you have a gun. Just make sure you don't get charged, because that's the biggest threat. Otherwise, Demoknights will basically be punching bags for your sentry.





The Heavy:
With a whopping 300-450 Hp and a "minigun" on him, the Heavy will deliver quite a bit of damage to your buildings unprotected before he dies. (Applied what he's fighting is a Lv.3 Sentry) Luckily, being such a big target means that more bullets will hit him from farther away, so you'll likely deal a full 180 crit with the FJ from slightly farther away, which gives you an advantage against Huo-Long Heaters. Apply the fact that Heavys are slow, and you got a giant bullet ♥♥♥♥♥ ripe for the picking.








The Engineer:
Finding an Engie as Engie is rare, and fighting an Engie as Engie is a tricky fight. However, the fact that you have a crit usually stored up means that the puny guy with 125-225 hp doesn't stand a chance. If you don't have a crit, then an Engie with a sentry nearby will easily mop the floor with you. If he doesn't have one, resort to the pistol or melee after you emptied the clip of the FJ.


The FJ Engineer:
This one really comes down to skill and loadouts. If he has a Wrangler/SC while you have a pistol, you're likely going to reign supreme. If you have a sentry or mini you can use, use the building process to distract the Engie while you get a few good shots on him. (This strategy goes for all classes.)

I think I'm running out of space, so I have to make another article for the other types I'm going to write up.

I was running out of space on the other section, okay?!

The Medic:
The Medic's only chance of survival is a random crit on his melee.

or if you suck at aiming

That's a little pre-judgemental to say, so going into more depth, the Medic has a whopping 12.3x more bullets in the clip than you of their primary, (If they have a syringe gun, otherwise they only have 1 medium power bullet) but they don't do as much damage unless the Medic has crits himself. Bob and weave the Medic's bullets to avoid his syringes if you're low on Hp. Crits do the trick, really. But his slightly mentally defective cousin has something to say about that.






The Combat Medic:
Combat Medics usually consist of the Crossbow, the Vaccinator, and the Uber-saw, with slight variations on the Medi-gun and melee. The crossbow does more damage/heals more on teammates the farther the shot came from to stab you, so C.Medics with good aim will usually try to snipe you. However, C.Medics still don't have much more HP than you, so the Sentry Distraction might work if you're 1-on-1 with him. Crits still do the trick on them. But I've been ignoring the main purpose of Medics, haven't I?


Ubers:
"Uber"view:
Get it? Uber means over in German? ok. Anyways, ubers will destroy your nest. Seriously, they will destroy your nest. However, some ubers are easier to defend against than others.

The Ubercharge: This is the most effective uber against sentry nests. The most well-taught Medics will uber while still having a patient and get as close to the sentry as possible to take the knockback for the uber-patient. It really depends on the circumstances, but the most common thing to do is to just run. Let the pair destroy the nest, and come back with crits later. That'll teach 'em!

The Krits: The second hardest to defend against, the krits won't do anything to your nest. But they will lay waste to you. The pair will usually follow up with the destruction of your base, which makes you lose all your crits that you otherwise would have gotten if you were alive.
I know, I know, it's painful. But it can still be avoided if you just cut your losses and run, then come back with a crit later.

The Over-heal Uber: Run away if you have to, but let whatever happen to your nest because if it was crowded before the uber, your nest will usually stay intact. If not, let the sentry be destroyed or destroy it yourself, and follow up with crits. The 300% heal rate won't be enough if you do a few full crit body blows to the Medic or the patient. Remember that knockback won't do anything either.

The Resistance Uber: This is a tie for easiest uber to deal with. The Medic has to choose a resistance. Fire won't do ♥♥♥♥, explosives don't do much either, and bullets can still be countered with rockets.



The Sniper:
Moving on to something else other than Sir Healsalot, the normal sniper will either pick off your buildings from afar or not do anything because you're in an enclosed space. If a Sniper from afar is picking off you or your buildings, move your sentry or purposely destroy your sentry and run. Don't try to get those 18hp shots off of him, it's not worth it. There's something another sniper can do, though.









The Close Range Sniper:
Close Range Snipers (who tend to have Huntsmen) usually try to pick off you rather than your buildings.
because what snipers try to headshot sentries?
It's easy to try and go for him, but other, higher DPS classes will probably take you out followed by the nest. Either that or he predicts your movement and headshots you. This kind of Sniper is usually easier to crit against since he isn't in a tower.








The Spy:
This class was made for breaching defense. If someone just sapped your sentry, bust a crit cap in the person. And if it isn't a Spy, congratulations, you just wasted a shot! Anyways, crits are super-effective spy checkers and killers if they're clearly spies. Even if the Spy dead rings, You still likely took a good chunk off his health. If he's running away, use the pistol if you have it on you (this applies to every class.) to not waste crits.

The Engineering Bible:
This covers just about everything you need to know about playing as engineer, including a brief overview of the FJ at 10:40!




Team Service Announcement: The Kritzkrieg:
Yes, yes, the video talks about the Kritzkrieg. But that doesn't mean that the video doesn't sum up the FJ in a nutshell (except it's a soldier and you have infinite time to use your crits.)

This is a visual excerpt from the video. Remember this both offensively and defensively, as it's a blessing and a curse for FJ Engies.

http://share.gifyoutube.com/KY9Vzj.gif

Remember:
You only have 3 shots before you have to reload.
While critted, you can deal damage equal to a multiple of 18 (up until x10)
You can only store 35 crits.

This thing:

Credit goes to TF2 Wiki for merc pictures.