in building Tristana, many players prefer to add armor penetration instead of attacking speed, personally I tried with both combination of runes, sum up with the conclusion that boosting up with Tristana's att. speed is far more useful in combat!

Marks: 1.7% x 9 = 15.3 %
Glyph: 0.64% x 9 = 5.76%
Seal: 0.76% x 9 = 6.84%
Quintessence: 3.4% x 3 = 10.2%

all together :38.1% boost up

My core item choices are :
1)berserker's greaves
2)bloodthirster
3)phantom dancer
4)infinity edge

5th item choice: phantom dancer.... indeed
with large damage 15% lifesteal is all your need, 5th and 6th items: are mainly to boost up your damage and attacking speed, usually I would add a second set of phantom dancer just to boost up its attacking speed and crit chances; with 2nd set of phantom dancer ... now you are looking at 2.154 in attacking speed, this is pretty darn fast, FYI 2.154 means in each second you attack 2.154 times to your oppoent; your regular normal hit is 283... not that much right? but your currently critical chances are 87%, with infinity edge unique passive which boost its damage up to 250% of 283, that is 707.5! with this build, for each second, you are attacking 1523.955 (707.5 x 2.154) in damage, this is pretty impressive!

now for your last(6th) item: it depends on your oppoent, if they have jax, get the swaord of drivine(unique passive: your attack CANNOT be dodged)

If not the case of jax, I would prefer damage items like Tiamat/ B.F sword/Frozen Mallet or defence item like Banshee's veil(block one negative spell)/Guardian angel(upon death restoring 750 health and 375 mana)