Alright so, I see several problems with this guide, some bigger then others. Let me post them all here so that you will hopefully fix them.

1. You wait till level 18 in order to get your third point in your ultimate. Tristana's ultimate is really good and much better then another rank of explosive shot. 1 rank in explosive shot does 30 more damage. A point in your ult does 100 damage. With 500 damage for you ult, you can use it to snipe people who have 4 or less health bars for an instant kill. You may be saying, "well your ult has a longer cooldown and is not useful for farming". This is true, but at level 16-18 it is definitely late game/team fight tine, and your ult is much better for team fights.

2. You get a bloodthirster before an infinity edge the build, but in your guide you say that build is not shown in the guide. (I personally think that infinity edge is always a better first item)

3. You should not be getting cleanse for tristana. For two reasons: 1. cleanse is a good spell, but if tristana got CCed then she is probally screwed. Even if you activate it almost right after you got CCed, the enemy team has probally already focused you to death. Plus, cleanse is next to useless on a team with no CC. So if you play blind pick, you might end up having cleanse more useless then revive. I am fine if you make cleanse as a "other possible summoner spells" but putting it as a main spell is just like always putting a thornmail in a build, cept worse since your tank should be taking all the CC anyways.

4. You are completely wrong when you say that there is no need for flash since you have rocket jump! I take flash on leblanc and she practially has 2 built in flashs. Flash is a faster spell so if you run into a trap you will want to flash out, not rocket jump. Plus, rocket jump isn't always off cooldown. If you use it for farming and then get ganked, you are screwed. Or if you face a leblanc, you will want an extra flash to stay away from her. Flash is such a great spell and should not be avoided

5. Tristana is an AD carry, not a tank. So you should really be focusing on offense masteries. You say that you want the final in defense but it doesn't even work well with tristana. The health will be bearably noticable since you only have like 2000 health at level 18 and you shouldn't be getting CCed. Plus, with your current build, you already have cleanse.

Most of my other problems are just only my opinion but changing those things would highly improve your guide. (Oh, and you don't really have much meat in your guide, look at some of the top rated guides and add some of the stuff that they talk about.