start with boots and 5 pots for sustain and harassment
then go for hextech gunblade and will of the ancients.
build rylais early for some x-tra tankyness and to slow enemys running away.
upgrade the boots for some magic-pen.
then you can build either liandrys torment, if the enemy is very tanky, and does minimal damage, or spirit visage, if the enemy is really high damage, and you need x-tra spellvamp to stay alive.
add a voidstaff, and as the last item, you have multiple choices: either build the standard deathcap, so you do extra damage on all abilities, the deathfire grasp, if the enemy team has lots of squishies, who you need to burst down fast, or the Guardian angel if the enemy has high burst and tons of HP