Sweet September

Pinkie Pie isn’t the only potion maker in the Five Realms. After discovering about another gypsy left alive after Bleapuroth's "purification period" when all mountain ponies and gypsies were banished, she is inducted into a potion battle, where the two must show off their potion-brewing skills and outdo the other: the loser is killed. She is led to believe that she murdered Pinkie Pie, but she was tricked and Pinkie is the victor over all.

Sweet September's Endgame
She is melted down like acid by a potion crafted by Pinkie, which was a very hard potion to create, especially in the timeframe in which the opposing mare had created it.