Autowin 6-mage Ahri Patch 10.20

Your win condition in this scenario is Ahri casting her spell once.
Yes, just once.
Even a 1-star Ahri will deal about 3k damage in an area 2 times (different areas), enough to clear the battlefield, and if you manage to get her to 2 or 3 star then it's really an autowin (I have dealt up to 42k damage with one spell).
Prioritize level 7 so you get the ideal 6-mage and a tank (2 tanks in best case scenario). If you get level 8 add in Morgana for that sweet extra MR reduction in an area.
Prioritize both crit items for Ahri. For a third item I suggest Blue buff so she casts her first spell quicker, or Hextech Gunblade, so she doesn't die in the off chance that the enemy survives her first spellcast. Tank items go for your tanks, attack speed items go for lulu or nami, so they use their abilities faster, and extra dmg items go for veigar, as a second carry.
Don't forget to swap TF for Lillia when you can.