"As Discordia, use your Golden Apple of Discord Ability to throw the apple to an enemy and have it spread automatically to 2 other enemies in a single cast."Golden Apple of Discord is Discordia's ultimate ability. You throw forward a large golden apple that will bounce off walls and damage minions. If it hits an enemy god, they are forced to hold the apple, until it explodes and does damage. When this detonates, there is an Area of Effect explosion (you can see a large yellow circle). If another enemy god is in the circle when the apple detonates, it spreads to them too.
For this achievement, you simply have to hit an enemy god with the apple and have it explode and spread to two different enemy gods. You don't have to kill them as far as I know, just have it spread to them.
I recommend doing this in Arena, as enemies are more likely to bunch up. I also recommend building cooldown reduction items such as Chronos Pendant, to help speed up how often you can use the apple. Also you could build Breastplate of Valour (you get extra mana with this item as well which is always useful as a mage), and/or Spear of Desolation (if you want a bit more magical power and penetration instead of protections, also the passive of this item takes 1 second off all your ability cooldowns if you get a kill or assist on an enemy god).
KBop a doo has kindly made a video showing this achievement. You can find it here: