As Geb hit three or more enemy gods with Cataclysm.
— Cataclysm is Geb's ultimate; it deals damage and stuns enemies around him in a radius of 30 units (roughly the width of the middle lane in Conquest).
Arena will be your go-to mode for this achievement, for enemies are often packed together there. Buy the Blink relic to engage from farther away and bring the element of surprise.