Titania. But it requires a little backstory.
You see, I was training mons for a possible Amaria fight, so I had an underleveled Gastrodon, Lanturn and Ludicolo on my team (all between 70 and 80, mostly on the lower side), plus an HM-slave (linoone I think). The only real team members I had were Metagross and Mist Mamoswine. (Okay, Ludicolo could take a small hit, use rain dance and, if lucky, take out one weaker mon before perishing).
I did not know where I could access a PC to change my team (I found out later). I had trained three fire types (Blaziken, Chandelure, Volcarona) and four ground-types (Mamo, Diggersby, Flygon and Excadrill) for this fight. (only the first in each was on my standard team).
Now, imagine trying to beat her with that.
It took ages to get through her first three mons. Then Mega Scizor happened. Adjustments had to be made, and many tries to figure out which.
Then Aegislash happened, and Mamo was gone. Metagross was left. For the first time, I used a revive in a gym match, sacrificing Metagross, hoping Mamo could take her out before it did himself. (Mamo had Ice shard.)
And it somehow did.
Shortly after I beat her (when I exited her realm), I found out where I could have changed teams.
Those weaklings I was raising stomped Amaria, tho. Storm Drain and Volt Absorb in that field is hilarious if you take out her Ludicolo first. So it was maybe worth it a little.