what a great guide, i've tried the item build and it really hurts :D. Can you please make a guide for Tryndamere as well?

this guide is incorrect on a number of levels.

First, Skill priority should be (R) Buster Shot --> (E) Explosive Shot --> (W) Rocket Jump ---> (Q) Rapid Fire. Prioritizing a point in your ult and then e and finally rocket jump before going into Rapid Fire. You only ignore this to put 1 point in rapid fire at level 7 to push turrets better as you should be able to get first blood once you have your ult if not sooner. Rapid Fire is prioritized last because the attack speed increase while large is minimally effective in team and 1v1 fights whereas Rocket jump and the healing debuff from Explosive shot are necessities in team fights. Also skilling up Rocket Jump will not only do more damage but put it on shorter cooldown.

Second, Ghost and Exhaust while not bad choices are inferior to Flash + Ignite or Teleport + Ignite. Flash/ Ignite is currently the best combo with Teleport/Ignite a close second as it allows for counter ganks and allows you to go back to base more frequently. The reason ignite is such a strong candidate is because of the Tristana combo. Rocket Jump onto an enemy, smart-cast explosive shot, smart-cast ignite, Once landed cast Buster Shot. This should be enough to get the skill. Especially early game. With Exhaust you will not have that combo, and without flash it does not allow you to escape assuming you have already burned your rocket jump or you used it accidentally. Rocket Jump is also still prone to effects such as stunning and snares due to the slight delay before jumping. You can also be hit with effects such as Ashe's Stun Arrow while jumping whereas that is not the case if you flash through the arrow.

Third, Runes are completely incorrect. As Tristana you want to be trading blows as early as possible to ensure lane dominance. That is why Yellow marks should be flat armor (resilience) blue should be flat magic resist (warding), while using Armor Pen reds with 2 armor pen quints in addition to one health quint. Mana regen is not a priority of tristana due to the fact that she can easily rack up minion kills without having mana and it is only a necessity when in prolonged fights which individuals should not find themselves in especially early game when they are most effective.

Forth, Masteries are slightly incorrect. Grabbing 4 points in Alacrity is unnecessary due to your Attack Speed steroid. Therefore Sorcery and the 15% magic penetration are both better as your buster shot scales off AP and the shorter cooldown should greatly aid with Rocket Jump and Buster Shot. The 2 points in Offensive mastery are completely unnecessary. While 9 points in Utility are not bad, those 9 points would be better suited in the defensive tree due to the trading blows. With 3 points in Resistance and Hardness and the other 3 in the second tier in Strength of Spirit.

Finally, the items are off a bit. The best possible items you can get are Berserker Greaves, Infinity Edge, Last Whisper, Phantom Dancer (x2), and a Bloodthirster. However, if the team is extroardinarily tanky you can apply Madred's Bloodrazor and Frozen Mallet to this build while dropping a Phantom Dancer and Bloodthirster.
- the 2 Phantom Dancers are extremely powerful candidates for BiS as it essentially makes your delay between attacks non-existent.

hopefully this helps. Here is a good resource: http://forum.morecowbell.com/default2.aspx?g=posts&t=274

not too bad not too good