This guide is a typical AD carry build but with the exception of extreme armor penetration. I intend to use this build if there is ever a case of high armor stats on the enemy team, this way I can shred it and carry the game as intended.

Pros

Your late game will be great!
Your ult has a very long stun duration (depending on the distance of the ult)
Your slow is really powerful when setting up ganks for the jungler

Cons

You may have a hard early game because of lack of damage
your passive is only useful when you do not auto attack for a set amount of time
Ashes ult can be hard to hit especially if you want the full stun duration
she has long cool downs especially her E (Hawk shot) and early game W (volley)

Make sure you focus mostly on last hitting minions as this build will be expensive, the passive on E (Hawk shot) will help as it gives you extra gold from killing units, it may not be much but after a while it all stacks up and help a lot. Looking out for the jungler every now and then will ensure the time to position yourself correctly for when he does come to gank. Always look out for the enemy jungler, relying on your support to ward the river and brushes etc. you wanna be sure to be over 100 CS (creep score) by the time the timer reaches 20 minutes. Ask your jungler for red buff as this will greatly enhance the damage of your poke on the enemy carry, and also stack on the slow you already have.

When you achieve level 6 and have your ult ready, be sure to poke the enemy ad carry down to a reasonable health. Once he comes back to farm up, aim your ult carefully, using your supports help and hitting your ult you should be able to pick up the kill.
Use W (Volley) when either being chased or chasing someone as it slows them and allows you to get away or catch up, with the help of Q (Frost Shot) they will shiver to the bone !

Be sure to always be at the back, behind all your team mates. this way you can avoid getting jumped on. Always be sure to use your ult on the correct target in a team fight as it can help win the fight. Attack anyone who is close by, don't try to overextend your range to reach a specific target as this can be fatal. Attacking the most dangerous enemies closest to you is the importance, use cleanse if you are caught of CC (Crowd control) and try to evade the range of the enemy, kiting him if necessary. Range is what your champion has, use that to your advantage. With the high amount of armor penetration you have, anyone tanky shouldn't be much of a problem.

You could trade the black cleaver for the last whisper. You get more flat armor penetration from the last whisper but you will lose the health bonus from the black cleaver, also, the black cleaver has more attack damage than the last whisper, I think its only 10, not sure though. They are generally very good armor pen items, the choice is yours. I would personally go for the black cleaver because of the health bonus and the extra damage.