Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Tristana is one of those heroes that can be one of two things: really, really good or really, really bad.
This build is how I play her, leading to 9 out of 10 games with over 12 kills and fewer than four deaths.
I'm not to big into runes, mostly because I haven't completed my own set, but the intended selection is crit dmg marks, atk spd seals, cooldown glyphs, and flat health quintessences. I feel these are the least important part of a build, and should be done yourself based on your own idea how to best work with the build.
I would say the normal 21-9-0 or 21-0-9 set up will work, but i like to splash a few extra into offensive and spread the remainder into both of the others. This is because Tristana is both squishy and mana dependent. If you throw a few points on armor, magic resist and mana regen, you cover all the bases and it makes early game significantly smoother.
I prefer using Flash and either ignite or exhaust. A lot of people thing flash is redundant on Trist because she already has a good combo of escape moves, between leaping away and pushing with Buster Shot, but it's still the summoner spell I use the most with ANY hero regardless. Its just that useful. As for the other spell, its comes down to how offensive you play. If you are willing to chase enemies but want an almost assured kill, take ignite. If you don't want to risk getting destroyed by a turret chasing someone, or getting ganked, take exhaust. You can modify the offense masteries accordingly as well.
Gameplay and Items:
You're gonna get Explosive shot and the Scythe right of the bat. The lifesteal is the most important thing to have starting off. If you decide to lane mid, harass with explosive shot, both passive and active. The active reduces all healing, including any lifesteal, and the passive will keep them moving away from your creeps to escape their own, preventing decent farming. From here on out, alternate between Rocket Jump and Explosive Shot when you level up. If you harass them enough, you can leap in with RJ, hit them with ES, and attack them as they run, usually killing them from half health or lower. This becomes easier at level 6 when you can add BS to the combo, or even push them into towers. If you do this right, compounded with the life steal, you should be able to lane straight through 2.5 k gold. I usually stay out as long as possible, so when I shop the first time you leave with AT LEAST boots completed and one part of guinsoo's.
Guinsoo's is a key part of this build. Why? AP+DMG+SPD. it is often overlooked, but Tristana's abilities have great AP scaling (1:1, 1/10 :1 per second, and i think 1.6:1) with the exception of rapid fire. as a result, you don't need that much to really make her tough. Once you get Guinsoo's, you focus on getting your damaging items: Blood Thirster and Infinity Edge. At this point, you can farm like no other, and in a team fight you are the heavy artillery. Just play smart: Tristana is SQUISHY! Always hang back a little if you know where the enemy heroes are, since your passive gives you more range than other heroes and you can volley over the clash of melee heroes. If you don't know where they are, stay in the middle of the group.
By late game, you should have Blood Thirster done and be nearing the completion of the Infinity edge. Once that's done, you can get the Phantom Dancer pretty damn quickly, and that really all you need to utterly whipe out most enemies. However, there is an open slot, which I use as a utility slot. Too many stuns ruining your day? Get Banshee Veil. Mundo refusing to die? Get Morted Bloodrazor. Casters pissing off your team? Get Wit's End. You can do a lot with that last slot if you need to.
Some other notes: You can switch out Bloodthirster with a few other lifesteal items like Starks, but I find Blood to be the best. As far as tactics go, remember you can approach quickly and with splash, but avoid leaping into the fray if your team's not heavy on damage, or stupid. Its always better to save the leap for escape and chasing, since you are a great ranged dps. If you're too squishy, get that fun ice hammer. Quite useful.
Well, that all for this build. It is my first one, so please critique it to hell.