If it is just Q upgrade to max, I would start with Deathcap, deathfire grasp, zhonya's hourglass, magic pen boots, magic pen Percentage staff, and liandra's torment.
Since people at the top comment requested the reasoning was simple all 4 items deal its most maximum on ap therefore you get a bigger bonus with deathcap, magic pen percent shreds over their existing magic resist plus more if they don't have any and amplify your damage, and Liandra's torment is for the magic penetration as well as hp shread for 5% if you are dealing with tanks with hp and magic resist.
Also spamming spells over time would mean that you would need a blue buff to continuously spam spells but I highly doubt you will be mana depleted with this gear type and whoever your opponent is fighting wouldn't die.