Hi, this is my first short advanced guide for botline Tristana with some support. This guide works, if everyone in your team is tryharding ( play current meta).
*I hope nobody blames me, because of missing orthography; just check the basic ideas.*

Why Tristana is the best carry?
1. very strong lining phase
2. unbelievable mobility
3. attackspeed steroid and highest natural autoattack range ingame

4. Runes
5. Masteries
6. Items
7. Skill Sequence
8. recommended Supporter
9. positioning in teamfights (!)

At first you need to know that abilities advance your line power, on the other hand some steroids buff your auto-attacks => buff ur lategame when your autoattacks deal much more dmg than skills.
If you compare all ad-carries, you will notice that these ones with much abilities or stuns / slows are stronger in lining phase. On the contrary you can still lose the botline ( lose your tower, die or have significant less lasthits as the oponents carry) if you get harassed to much. If you play against strong harasser like Ashe, Kogmaw, Urgot, Caitlyn you need a offensive support ( watch chapter 8 ) to slain enemy ad carry if he overextends.

Tristana ( and Ezrael ) are the strongest lvl 2 fighter at botline. So try to reach it fast by pushing the line (always autoattack the minions). Always try to push your line, so your supporter is able to ward and you are able to push enemy tower and enemy ad carry lost some farm.
Furthermore Tristana is really scary at level 6 cause of extra ulti burst.
If you start at blue side, ALWAYS ask your jungler if you ( and your support) are allowed to take double golems.
The point is: If you know you are stronger and you are stronger with higher lvl( and Tristana always stronger at lowlvl, especially with heal / ignite :)) engage at enemy ad carry. So you able to to kill him and win the line, or force him to go home and win the line, or force him to stay back and win the line because he lost farm and lost tower.

Tristana's Rocket Jump ( w ) is one of the strongest abilities in the game, because its the longest position chanching ability in the game, you deal decent dmg and slow the enemy.
Also you are able to jump again, when some enemy is died.

In addition you can use your LEAVE-ME-ALLONE-ultimate to shrug annoying tanky-dps off.

So later with quicksilberdash (qqs) and your enorm attackrange, you should be unable to die, if your team does not sleep, or you are too greedy. :)

Nothing to say, just too strong with point 1 and 2 :X

"Win ur line, win the game" is the current meta.
So you try to improve your lining power with your runes.
Attackdamage-quints, Armorpen-marks and armor-seals are pretty standard.
In some cases you dont need flat magic-resist-glyphs, for example vs. soraka support.
In this cases you can take per lvl magic-resist, or attackspeed.
Tristana dont need manaregg if you play decent supporter, because you will burst enemy down.
Champions with harassing abilities just need manaregg. ;)

21 points in offense are pretty standard for all ad-carries, because you want to deal "tons of damage". I can add you need Lethality (+ 10% critdmg) because you always buy infity edge (chapter 6).
9 points in deffense just more improve your early game.
Tristana dont need manaregg if you play decent supporter, because you will burst enemy down.
Champions with harassing abilities just need manaregg. ;)

Why Tristana dont need flash?
Read chaptor 2.
If enemy team is full of stuns, which would kill you, take cleanse instead of heal.
(CARE: you are not able to cleanse warwick ultimate, skarner ultimate, Urgot ultimate (!) )
Always take Ignite, if you know how to play Tristana because you need the extra burst.
If you need a exhaust in team, first ask other mates to take it, you need ignite. (!)
If you dont know enemy lineup take Heal, ignite.

You could take Ghost instead of heal, but in most cases you dont need it because if your big mobility.

Just remember: Champions with small attackrange should buy Bloodthirster instead of Infinity edge. Champions with huge attackrange should buy infinity edge instead of Bloodthirster.
Why? Because you deal much more damage with infinity edge, and with large auto attack range you dont need much sustain. ( exception Ezrael )

Tristana always start with Dorans Blade, to be stronger at earlygame.
The trick at shopping is: Always buy much as you can and wait 15 secs at most for some money.
Never sell items, if your inventory isnt full.
For example: If you need to go base with 780 gold, buy boots and vampiric-scepter.
If you got 1400g, go for Beserker-greaves and second dorans blade.
If you save 2000g, go home for BF-sword ( BestFriend-Sword ) and boots. (perfect first shop!)

Infinity Edge always should always be your first combinated item after Beserker greaves.
After Inf edge, you can improve your dps alot with 1 zeal.
Now you need to choose, if you need
a) Last whisper vs much armored enemies (dont forget soraka heal),
b) Phantom dancer for more dps vs unarmored enemies and more mobility.
c) quicksilverdash if you able to die at stuns or exhaust.

Always buy a) b) and c).
If you miss sustain at line or vs. anoying tanky dps or thornmail, buy 1 vampiric scepter.

Your last item should be,
- Bloodthirster,if you are realy not
- Phantomdancer,if you need to kite (care you are without lifesteal, but heal /
supportassist could be enough)
- Banshees Veil,if you are scared of burst damage (care you are without lifesteal, but heal /
supportassist could be enough)
- Giant's Belt, if you havnt enough for banshees veil :)

ALWAYS ALWAYS after Infinty Edge + Zeal and you cant buy your next big item, buy red and green elixiers.

If you gonna lvl 1 fight, skill Rocket Jump because of Chase, chase, chase.
If not Skill "E" to push harder, to reach lvl 2 earlier than oponent.
Max "W" first, because its your op-spell.
At lvl 4 skill "E" second time, because of stronger burst.
If you start skill "Q" at lvl 8 you finished it at lvl 14.
At lvl 14 you should have infinity edge (~ 20 min ).

You can say: R > W > Q > E wenn lvl >= 4

If your mate pick supporter, he should try to counter enemy team first.
For example pick Janna vs Kennen / Amumu; pick Alistar vs Galio; pick soraka vs karthus; Leona, if your team need a tank and opener. Sona always good choice. :)
Tristana can line with every supporter, but he needs to be offensive if you are offensive.

For current Meta supporter DONT need flash / cv. In most cases exhaust is more important, because the cooldown of flash / cv is too long. And you just need it, after a mistake. ;)
Only Alistar still needs flash, if you ask me.
So I suggest Heal / Exhaust / Promote / cv for a supporter. ;)
*edit: Use promote when enemy ad carry is back to base to push tower, ur use promote when own carry is shopping to deffend own tower.

You need to know: You are the most important player in your team. Steal every kill you can.
Sacrifice everyone in your team to survive, especially your supporter. Take all farm you can.

Always stay behind your tank, and in near of your supporter. You are the fírst adverse target. So you try to survive and kite them. First jump in if you are safty. Generally in teamfights always use ur "W" defensive. Now your automatic autoattacks deal much more damage than your spells.