Not Updated For Current Season
If you find yourself being killed quickly by magic damage you can buy Abyssal scepter after deathfires grasp or if its a heavy magic team go for lich bane as well.
Zil is very good early game for harrasing so push hard, with clarity/doran's you should be able to bomb a target every time it comes off cooldown up to lvl 9.
At lvl 9 you run out of mana quickly so save your mana for team fights or your ult.
Your spells for nuking a champ should be bomb, rewind, bomb. (unless you have deathfire witch you should open with that)
in team fights try to save your time warp to keep there champs from running away!