What you want to do is just walk up to them auto w and keep autoing. This works in all elos except for high diamond+. Most of the time you will get first blood and flash because your corrupting potion and your passive does amazing Dot (damage over time) And when you get 5 stacks with your passive you deal so much damage.

When you start a team fight locate a very squishy target. It doesn't matter if it's the support and kill them with your ultimate. This will give you 5 stacks after any attack and this makes you deal the most damage possible. Then hop onto the most dangerous damaging character and kill them and deal as much damage as possible while drawing as much attention as possible.