Volume 7 Weiss is a lot stronger than she was in Vol 6.
In Volume 7, she was keeping up with Winter. Also, there isn't a delay in her casting glyphs. She managed to get off the Atlesian Manta and cast an Earth Dust wall in less time than it took for Cordovin's cannon shot to reach the main cast. Even summoning, which takes significantly longer than regular glyphs, is effortlessly nigh-instant for her by Chapter 9.
Weiss can tire Adam out through repeatedly encasing him in ice, forcing him to smash his way out. Weiss is also faster than Yang, who could clearly react to the Moonslice attacks, and the Arma Gigas is strong enough to defend against them. Weiss also has flight if she uses the Queen Lancer, or even regular Lancers, so she can easily play keep-away.
Weiss' stamina is also clearly underestimated here. In Volume 2, she had a full day of fighting Grimm, a sleepless night followed immediately with a race against time to stop the train, getting knocked out by the WF lieutenant after a fight that made heavy use of her Semblance, getting knocked unconscious again by a train crash, then finally fighting off the hordes of Grimm during the Breach. Weiss came out of all that with zero signs of exhaustion, plus she's definitely got more stamina after weeks of training with her sister over the course of the Volume 7.
Adam's a one-trick pony. He'll try to take hits and dish them out with his Semblance. Weiss has experience sparring with Yang's Semblance, so she'll know how to deal with Adam's. She'll evade him and tire him out, and when he's exhausted, she'll overpower him with the Arma Gigas faster than he can fire back with the stockpiled damage.
Basically, she'll Neo him.
Voting Weiss.