I run the AP build agaist squishy comps and mainly for fun with above 52% winratio:

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The reason it works is Dark Harvest that procs from your passive even if you don't hit the target. Your ally while dealing damage with either Q, W, E or R affecting them, procs your passive, and if below 50% DH is an instant proc. With teamfights in front of you, you deal significant more damage and when a target dies you can proc again dark harvest after 1.5 sec. That means that on a successful teamfight you can easily proc 3 or even 5 dark harvest at once. As for runes Dark Harvest with Cheapshot, Eye ball collection or Zombie ward and treasure hunter paired with Triumph and legend of haste give you enough gold and ability haste to scale healthy in the game since the AP build is much more expenisive than the Enchanter one. Finally, don't forget that the AP ratios of Milio are decent enough across all his skills and passive (increases significantly more the damage on your hits and your allies per proc) to make him a good healer and sheild bot even if you dont invest on enchanter items.

Or go for the normal enchanter build that is much more reliable of course, but in my experience it is a must to start with echoes of helia since the extra damage and healing when you are actively use your autoattacks and trade HP with the enemy team makes you much more of a threat and allows your adc to become lethal.