Assuming that you are in level 18, activate "IMPURE SHOT" first, then you start hitting him wih "DOUBLE UP" then you use "MAKE IT RAIN" to slow him, and then normal attack.
This skill sequence is very useful agaist champions who dont have too many health, but when you get the items you can slain everyone, even a tank.
The ulti is useful when you are on a team fight, when all the opponent team is in front of you and all together, this maybe can slain 2 guys.

We start with the Doran's Blade because of the damage, when you get the B.F. Sword you are gonna hit 250 to minions and 107 to champions with the "DOUBLE UP", and 47 each bullet to minions with the "MAKE IT RAIN", this skill is very useful when you are escaping from someone. The Infinity Edge will give you crit. chance and The Bloodthirster will save you in a battle 1v1 because of the lifesteal.