Azeria

The seed makes her Quagsire typeless, and gives it +1 defense and +1 special defense, this coupled with unaware and amnesia+recover quickly turns the fucker into a juggernaut. In both encounters I've had with it, it took me a good while before I became able to stop it, both runs running grass starters in Sceptile and Decidueye respectively(The latter being a ghost mono), and then I still had to deal with best mon.