First you are going to want to grab boots and 3 health pots. Then you need to get sorcerer's shoes, unless you have saved enough money to get hextech witch shouldn't happen. Once you get your hextech it is basically gg for them unless you fed. And then rabadons you just OWN them. Getting to the optional, if you don't want much life then get a void staff deathfire grasp and if you need to, get another deathcap. But if you need health or MR or armor just get all the other items. Abysall for MR zonyahs and guardian for pretty much not dying, and then warmog if you need situational health.

Once you have your Hextech or Rabadon I reccomend this. Let your team start to get them down then just jump in with your Q,R,E,Q and then q and r for the hell of it. And if you need to escape then use your w to juke them. If you are still in laning phase and only like lvl 6, if you have a hextech and they are below half health (assuming your above half health, or not diving) then you need to do pretty much the same combo. Q,R,E,Q,R and then keep doing that till they dead and then use your w to get away.