Tristana is always best with a tank. So if you have a tank on your team then I would most likely go with him/her. I wouldn't reccomend going in the middle with Tristana, I would mainly go top or bottom on your map. Now when you come across a enemy dont always go for the kill, just start off by shooting them and if you can try and hit them with Explosive shot. Now what i like to do is just shoot at the minions but if an enemy attacks me i will normally attack back or you can just kill minions to level up until you see the outline of the enemy turn yellow and that means their getting ready to attack you.

When i made this build i thought i should try and go for attack speed before damage. So i tried it and i did pretty good. After you buy the Phantom Dancer and Berserker Greaves you should have 423 movement speed and about 1.5 attack speed. After that its all about damage and critical strike.