As Hachiman, hit 5 enemy Gods with one use of his Ultimate, Mounted Archery.
This should be fairly straightforward, as it only requires hitting 5 gods, rather than killing a certain amount. I recommend doing this on Arena, where enemy gods are more likely to bunch up, making this kind of shot possible.
Hachiman's ultimate allows you to lock onto all enemies within a long cone area (see the picture attached) and an icon will appear above each enemy when you are locked on.https://ibb.co/nuXCQb
Approaching the enemy team from the side or the arena from the buffs gives you a bit of cover and should line most of them up for you to hit.
Plus you can aim your ultimate independently from the direction in which your horse travels.
You can do this as soon as you hit level 5 and put a point in your ultimate.
The only items I'd suggest building especially for this achievement are cooldown reduction items, such as Gladiator's Shield, Ancile and Jotunn's Wrath etc, so you are able to use your ultimate more often.
My build when I unlocked this was more damage/ crit/ attack speed based, and consisted of Warrior Tabi, The Executioner, Deathbringer, Bloodforge, Wind Demon and Odysseus' Bow.Edit
: another way to get this relies on a lot of luck, if you play Assault and are lucky enough to be given Hachiman as your god (or are able to trade gods for him), then you should be able to get this achievement fairly early on in the game. It’s only a single lane with few places to escape.