Exit the game and close its processes before the round is complete, but after you see the chest. The devs don't want the turn to happen out of your sight- don't want half your lineup to lose all their HP without you knowing how, I guess, so the turn resets from where you're about to choose your attacks.
Ideal is: Kill Atalanta with regular attack, transition into NP after she dies, you'll get a good window in which to see the drop and then exit the game if it's a bad one.
Edit: NOPE NOPE NOPE. All wrong.
Withdraw from the match, then retry. That's the only way. You can't actually get different drops from redoing just the one turn.