SMITE

As Charybdis, hit 3 enemy gods with a fully charged Capsize in the same fire

Capsize is Charybdis' second ability.
She absorbs water from around her before firing out in a cone in front of her. In order for the ability to be fully charged, you need your passive, Raging Tides.

As you hit enemies with your basic attacks, your Raging Tide increases, shown in the passive meter. When your passive is maxed out, you'll also have an additional attack speed buff. You need at least 405 of your tide meter to be able to fully charge Capsize.

For this achievement, you need to charge your passive to at least 40% by hitting your basic attacks. Then run up to a group of three or more enemy gods, hit your 2 to allow it to fully charge (you'll see the cone in front of you fill up). Once it is fully charged, make sure there are three enemy gods in the cone, who will each receive damage.

As long as you fully charge the attack and hit (you don't have to kill) three gods, this will pop.

I also recommend Arena for this as enemies group up a lot.
I'd recommend going for the currently popular bow build, to increase your power and attack speed, which will also help for your ultimate. The quicker you can build up your Raging Tide, the more often you'll be able to have a fully charged Capzise.

Also, remember that you don't have to kill enemy gods, only hit them.