I completed this challenge against someone with just 1 champion in defense.

I believe you need to go first and you need to kill right away.

So no boost skills like Athel's Higher Blessing. Because that already puts you in turn 2.

So go first and only press 1 button.

Hello, Roder.

There is a bug right now that prevents the quest from being completed if you have a Shield Artifact Set buff on your Team during the battle. It's something we are looking into right now, but the quick work-around is to fight without that set.