After spending 100 hours or so on this game, I found it odd I still didn’t have this achievement.
You need the Defend Harmony augment to obtain. Problem is when you do have the augment the computer avoids attacking.
My workaround is use two controllers for Local 2P.
I chose All For One (red, a single endurance battle) as it gave more augments each game. Defend Harmony is decidedly rare. It took me around 20 restarts to finally get it.
Once you have the augment, align your defense so both slots match (5 and 5, 4 and 4, etc). This will double your defense. Your total defense will reflect that on the side. Now have your second controller attack and achievement will pop.