First you buy vampiric scepter then you can chose side line or mid.
You rock at all lines, at mid you can lvl faster to gank with your ulti the side lines or
at side lines you have a made who can help you with kills something like garen or ashe with ulti etc.Later you buy berserkes greaves for the movement and attack speed this push your dmg output a little bit.Then you buy sword of the Divine cause it brings really high attack speed Boni and you enemys cant avoid your attacks and after 4 hits the enemy get 100 extra dmg that really nice with your 3 spell.Then you build the bloodthirster with your vampiric scepter for the dmg and life steal.Then you buy Infinity edge for the dmg,crit chance and crit dmg at this time you make so much burst dmg that caster cant win against you.Then you buy Madreds Bloodrazor against tanks and melees(at this time no one can win a 1on1 against you or 2on1 cause you crit so often that 600-900 crits came).Then the game at this time not end you buy a second bloodthirster cause the passiv stack and you have so much dmg (400-500)and lifesteal(30%-45%)that you can win against all.In the game you can buy elixir like aglity or fortitude for a little more dmg output.