You can get this on the Offline Local Mode since all the characters are unlocked there, select a character with a strong Down Air (Left Stick pointing Downwards + X or Square), the characters i think of from the top of my head that fall in this category are:
-Bugs Bunny
-Jason
-Superman
-Banana Guard
-Finn the Human
Select Iron Giant as the opponent thanks to his massive hitbox, take him off stage and use your Down Airs until his dead, make sure to set the AI to easy and repeat this until the achievement is yours
You may also get the achievements for getting Bottom Ringouts