ok here we go
21/9/0 - you wont have mana problems - just mana burst problems that utility cant solve anyways
start q,e,e,w then max r>e>q>w
Look for constant q poke to wittle their hp bar and to stack your passive.
Use your E to last hit sometimes to keep your mana up
At level three you can go for a cheese if they have about 75% hp. Q>EEEEEEEEEEEEEEEEEEEEEEEE ignite early. To pull this off get closer than usual to them before you q. Firstblood.
You have to pay very close attention to the duration of your poisons on targets because one mistimed E on a poisonless target results in 5 seconds with almost no damage. Its awkward.
After level 6 there is a neat move you can do. Stand outside of range so they feel safe then cast R>flash not Flash>R. It has a wind up and it basically always works. From there W>Q>EEEEEEEEEEEE and yeah.
In team fights look for the nearest cc'd target and Q>EEEEE. You should be strong enough that this kills them.
Build is:
dorans 2 pots>tear first back
ludens
rylais / sorcs
void staff / arch angel
situational