Love the item build you have goin here. Pretty genus but i think the talents could use some work like why Offensive Mastery and Alacrity when you can get Intelligence and Presence of the Master?

Alright sounds good.

when im going to kill someone or gank with the team i put make it rain to slow and then use impure shots with the ap stacking up u will nuke them little by little.. then if you see that its getting hectic or a fellow teamate is using his ult for example amumu or nunu gangplank then go ahead and use your ulti.. you will most likely get the kill since you have a DoT on them make it rain is constantly hitting and your ulti hits really fast so you will get the last kills.

I will give this a try, also what is the exact skill sequence you go by and what is your killing combo?