I just beat Plumeria on the Battle Tree in what is probably the longest battle I've had against her. It came down to a long stallfest between my Vaporeon and her Toxapex, and eventually we ended up exhausting all of our moves down to Struggle. I won because Vaporeon had managed to burn Toxapex with Scald beforehand, and because Toxapex's only offensive move was Liquidation, which kept healing my Vaporeon.