The achievement can only be completed if another player attacks you offline at the arena while you're using the Stadium postcard. Only players within the same league as you (Bronze, Silver or Golden) can attack you. If one of your friends challenged you from the Friends menu, their loss would not result in you getting the achievement.

So, at the

 red crystal in an arena, equip the stadium postcard, then save the config and make sure you are using the config in the Arena Configurations menu. You'll see the name of the battlefield you are using between the power icons and the arena perks icons, make sure it says "Stadium".

Once you've done that, the only thing you can do is wait at this point. If it takes a long time to get the achievement, consider improving your heroes' class combination and AutoBattle settings at the red crystal.