Tristana Flex ---> Masters

Getting to Masters this set took eternity, but I did make it, largely off of the back of this comp:

Here is a pretty basic Trist carry level 6 board. Janna is not consistent because she's a 4 cost, but all of the other units are 1's/2's.  Not pictured is the 3* Ziggs on my bench to force the Yordle Trait to generate 2/3 gold a turn.

Trist comps rarely if ever win, but consistently top 4.  According to my lolchess, 85% of the games where Trist is on my end game board, I top 4 the game.  High-end Arcanist/Challenger/Fast 9 legendaries boards typically edge you out and push you into 2nd and 3rd.

Why Trist?

This comp is very strong against Melee units, Trundle, Yone, Warwick, and yes even Katarina.  Trist's 3* ult knocks back 3 hexes, and if you can get the fight into overtime, Rageblade means that you instantly win against any melee units; they can't close the gap.

Consistency is your second big benefit.  As long as you can get Yordles active early (no later than 2-5), the units find themselves, and the extra money acquired allows you to transition into other comps/builds as needed.  While trying to play Trist, I find it easy to get into Arcanists or other AD carries, Jhin especially.

Carousel Priority

Unit is more important than your starting item here, as the earlier you can put Yordles together, the higher your chance to succeed.  Ziggs/Poppy are of equal value, the tiebreaker being if they have an item they could use, Poppy with Belt/Chain, Ziggs with Bow/Sword.  Even though Bow is the best starting item, I would only take Bow on a non-Yordle if it forced me to take Tear, this comp can only use Tears for Redemption or Shojin Cait.

Starting Item Priority after unit:

Bow > Sword > Glove > Belt/Chain > Cloak
If you start Tear or Rod, do not play Trist.

Bow Ziggs/Bow Poppy are optimal starts.

Socialite

Only play Socialite if it's in the back row.  Moving up a hex hurts Sniper, and is only good if it's one of the far side spots.  No other Socialite hexes are good for you.  Always try to scout the Socialite Hex early, when it's in a good spot Taric is an insane unit.

General Strategy

95% of your games you're going to try to lose streak.  Put in whatever units you can to make Yordles at 2-1 or fish for it in the next few rounds.  Don't put items on Ziggs because they're going to get stuck on your bench.  Don't feel bad if you take 38 damage on Stage 2, once you stabilize you are hard to stop.  Don't level early unless you are streaking/can streak, typically this means you spiked an early Trist/Poppy 2 with a Rageblade.  Level to 5 as early as you can without breaking 50, sometimes this will be at Krugs, sometimes this will be at 3-1/3-2.  

Trist 2 with a damage item is enough to stop taking 10 damage every round, as long as you have Poppy 2/Blitz.  Runaan's/Rageblade are highest priority, IE/DB/GS are decent but not as strong early, HOJ/Last Whisper are pretty terrible, none of the units you care about killing have more than 50 armor and HOJ is just a terrible item that no one should build unless forced to.  Runaan's helps you not get stuck on Vex against Arcanist players, and Rageblade + a Healing item allows you to outrace.

This is your level 8 board, try to end up here.  Tank items on Poppy 3, Damage items on Tristana 3, extra carry items on Jhin, Utility on Lulu.