This achievement is story related and can't be missed.
This is an unwinnable fight at the start of Chapter 11. Basically, the more you try to hurt Thanos, the more he multiplies. In order to "win", just give up and let him pound you.
However, this is a great opportunity to use Rocket's Five Barrel Barrage, since you fight an unlimited amount of Thanos clones until you lose (win?) the fight. Just kill a few, reload, rinse, and repeat.