Don't be mislead by the title of this guide. This is a serious guide, not some "play Tristana as tank"-fun-guide.

Tristana's purpose is to acquire items that allow her to obliterate the enemy-team and their buildings within seconds. A carry.

This guide is about getting there in a very effective way by playing defensive and then executing it in a fashion that will very convincing.

As most guides it is about teaching new players and players willing to learn on how to be more successfull. It will contain little news to very experienced players.

The aim of the Rune-choice is to provide a maximum of efficiency.

For reds it's out of question that Armor-Penetration is the best you can get.

The choice of all the others might be less obvious and that's why I'll explain them:

Why Armor and Magic-Resist for Yellow and Blue? The answer is: Those are the best scaling runes for their colors. They will provide the most significant bonus in lategame by reducing incoming damage by roundabout 20% for each damage type.

Why Health-regen Quintessences?
Most of the Qunits are merely 2-3 times better than the second best rune with the same effect.
For example: Armor-Pen Quints giving you 3.33 where the red ones give you 1.66.
Well, the HP-Reg quints give you 6 times as much HP-reg as a yellow HP-reg-rune!
Is that reason enough to take them?
Well, your goal is to never leave your lane for the entire duration of the laning-phase and thus achieving an advantage in farming and leveling. Over the time of 15 minutes those runes give you more than 2400 health back. This is very significant in allowing you to stay on your lane for as long as possible!

There's nothing too special about the masteries as displayed.

Besides maxing out the Offensive-Tree the side-focus is on survival, lasthitting and regeneration.
This shall explain why ghost, minion-damage, magic-resist, armor and regeneration were taken instead of the possible alternatives.

First of all:
It is very important to not see an item-build as set in stone!
Depending on enemy-champions and how the game fares the desired items or their order might differ!

For example I often start with a regrowth-pendant instead of Dorans Blade when the guy I'm facing while laning is a very annoying harasser like Karthus or Cassiopeia who have very long range on very cheap harassment spells that are hard to doge.
Or you might start with a Dorans Shield for extra-survivability if you are on a Duo-Lane facing opponents that get strong kill-combos.

But let's consider facing another range AD-carry midlane as standard.
Dorans Blade is very good to help lasthitting while providing health to survive unexpected occurences and some form of regeneration.

Next thing you get is shoes.
The added speed of shoes are helping you in any aspect of the game.
Again depending what you are facing you might consider getting Mercury Treads instead of Berserker's Greaves.

Ok, now it get's special. Remember the guide's name? It has to do with your next item. The sword of the Occult is a relatively cheap item that scales with your ability to not die! Everything comes pretty much back to this. The runes, the summoner spells, being friggin Tristana with her awesome escape-abilities. Who if not her is destined to wield such an item?

The next thing to get is the Phantom Dancer. Reason being it increases your speed which helps in not dieing and it consists of several cheap items that mean continuous improvemnts.

Continue with the Infinity edge to boost your damage to where it belongs.

Now that's the point where the enemies should start fearing you more and more and your kill- and assist-count should start cranking up. Most average games will end without allowing you getting further items.

If everything is going well:

Grab the Frozen mallet to unallow you opponents from fleeing and making you a little less squishy.

Still not done yet? Sell your Dorans Blade and get the Black Cleaver to make your damage astronomical.

If it is tough:

Get defensive items after your Infinity Edge.

This skill-sequence is pretty much the same as in most other AD-Tristana-guides with the single difference that it gets Rocket-Jump before Explosive Shot.
This won't be changing anything big but allows you to escape if your premade-team tries to provoke a level 1-teamfight.
If you are sure that nothing like this will be happening you can grab explosive-shot first.

Not getting hit at all is the main-purpose of the whole guide. Thus Ghost and Flash really are mandatory.

Enemy flees with low HP? You have no Ignite and no Exhaust?
Well, as long as you don't die it doesn't matter!
You can still farm more while they are gone healing up.

This is the most important part about this guide.

All runes, items and masteries won't be helping you if you do not play it right.

Laning-Phase:

As a carry you obviously want to go for the mid-lane. If for some reason you are not in a solo lane, inform your partner, that you will play very defensive so he does not overextend.

Walk in a random manner behind your minions and snipe the ones with low HP to lasthit and prevent enemy skill-shots or harasses to hit you as good as possible.

You will be automatically harassing your opponents by just lasthitting as you deal some splash-damage when you kill the minions.

Getting close to them to apply your DoT from your E-Spell usually is not worth it as you come in range of their auto-attacks or gap-closers when facing melees and get the aggro of their minions. But don't hestiate to do it anyways whenever your opponent overextends himself and gets to close to you.
Never autoattack them if you could get a lasthit on a nearby minion as this will damage him anyways and give you the gold for the lasthit.

You will want to be fighting near your tower to be able to escape there with a rocket jump, you might, however, if you have both your Summoner-spells ready and/or there is no jungler to worry about, push them into their tower to deny lasthits.
Whenever you come too close to your tower, just autoattack the minions and even use Rapid fire. Because of your Explosive-Shot you should be able to push better if required.

A lot of stuff here comes down to experience. Having played the Champ you are facing helps a lot in knowing what he's capable of and what not.

The ideal situation is that you can harass your opponent enough with your explosive shots to drive them back to their base. After doing that 2 or 3 times you might take out their tower.

You might say: "Wouldn't it be more ideal if I kill them?"
I answer: Be realistic! No decent enemy will die to a defensively-runed AD-Tristana before she has Infinity Edge!

When you kill their tower or lose your own (Yes, this might happen against some really nasty champs when noone helps you gank them.) the laning phase is pretty much over.

Mid-Game:

Ideally you haven't gone back to your base all the time and have saved up over 2k gold.
Now is the time to spend them.

What you do now depends on what happened so far.
Are the other lanes still all undecided? Was your lane the last to decide? Did you win or did you lose your lane? Do you have a dedicated jungler?

You won your lane and the others are still fighting and you don't have a jungler and your laning opponent stood on your lane? Push your lane back if the enemy-minions or enemies themselfes come close to your tower and go jungling if your minions are closer to their 2nd tower.

Under no circumstances shall you continue pushing your lane now! This is way to dangerous!

Another option is to help the bottom-lane and push ther with the goal to also get the dragon. Consider taking wards with you if you can afford them, as your laning opponent is very likely to follow you.

Generally play it save and never advance in enemy territory if you don't know where all of the enemies are! Warn your allies if they should do this! Be a role model by not following them in their doom!

Once all 3 lanes have been decided or even during that the game will develop into the next phase.

Team-Phase:
Encourage your team to gather up and walk around as a team. Ideally you will want to avoid 5vs5, especially at their tower. Instead you will want to have have 5vsLess.
Have your enemies guessing what you are up to. Don't just push middle as this will be what they expect.
Lure them out into the open by letting them push middle and then come from behind. Kill creeps, spread wards, just continously roam around the map and strike when it is not expected all the while sticking together as a team.

Unfortunately you will not always have your allies behave that way in solo-queue.

Now from the team-perspective back to your own.
Do not go in front of your team. Stay behind, but also be possibly not the one at the very end of the trail. Savest position is in the middle with a tank at the front and a bruiser at the back.
When an engagement emerges get into position before attacking anything! In position means to get your whole team between you and the enemy and possibly with you also being closer to you closest tower.
Now you have to follow a priority in what to do.
Number one priority is: be and stay save!
Only attack people that are already fighting while staying at maximum range.
Target as following:
1. People that are close to death no matter what they are. Not getting the assist or even kill is bad as you lose gold and a stack for your Sword of the Occult.
2. Squishy Melees. I think it is really obvious why.
3. Stupidly positioned Ranged/Support. Normally they shall not be targetable for you when they are following the same positioning guidelines that you do. If, however, they do it anyways, feel free to punish them.
4. Tanks. They are there to protect smartly positioned ranged and support. Let them have that work if getting to their ranged would mean becoming a "stupidly positioned ranged" yourself. In this case fire at the tank.

If your melees/tank dies quicker than theirs your only valid option is to flee. Even if there's some low squishy you could get. Do not risk your own life!

If their melees/tank die quicker than yours, you can try to chase down their fleeing rangeds. Or just continue pushing and let them go.

If someone, usually one of their melees and/or their tank, get's a jump on you during a team-fight, do not hestitate to burn all of your defensive cooldowns to get away, buster shot, rocket jump, Ghost or flash. Whatever is needed to get rid of them. This shall actually help your team greatly as the guy that came to kill you should not really be able to deal a whole lot of damage while your team continues to damage his team.

If someone of them goes for another ranged in your team, save your mate with a buster shot!

Save this thing for defensive purposes unless the fight is decided into your favor. It won't do a lot of damage anyways since you're not AP. It is there for the sole purpose of saving you or another squishy!

As soon as the person chasing you changes his target or is forced to do so by your buster-shot, turn around and continue with the above priority-list.

About your abilities when fighting:

Well, Tristana isn't really a Champ that requires refined spell-combinations in order to use her full potential. You basically push Q when you start shooting at someone and Smartcast-W your way out if someone comes too close to you. E's active will be pretty much useless later on and your Ult is just a situational lifesaver as described above.

Positioning and fleeing are the important traits during fights.

I want you to get into a certain mindset:

You shall feel really bad for every death you suffer.
Not dying shall be priorized over killing a lot.
You will automatically become a killing-machine that can kill without taking risks if you are just patient enough!

Also do not blame your allies for not saving you, when you die! You have more than enough at your disposal to save yourself, if you play it right.

Even if your team is really bad and you cannot do anything to prevent them from losing, I want you to have a friggin' Zero between your kills and your assists, even if your kills and assists are zeroes aswell!