When asked to stop Ace from running away in the prologue, Deuce asks, “freeze his legs? Or bind them with a rope?,” so it is possible that those are two forms of magic he is capable of, though we do not see them at the time.
Deuce settles on summoning “something heavy,” which ends in a cauldron.
Deuce says he “wasn’t expecting that,” but during White Rabbit Fes we learn that he would actually summon cauldrons so much pre-NRC that he became infamous for it.
In the novel Ace asks why it is that Deuce only ever summons cauldrons, and he says that in the heat of the moment they are the only thing that come to mind, because when he thinks of mages he thinks of cauldrons.
Deuce summons rabbit-hear headbands during the White Rabbit Festival.
Deuce says in Book 1 that he would show off his magic to other people back before he enrolled and decided to become an honor student, but does not specify what, exactly, he would do.
Deuce can also drive magical wheels, and has been doing so since before NRC. While it seems that they do come up with back-up batteries, Deuce says that they use their rider’s magic as fuel, so he likely uses magic to drive them.
Other magic we see Deuce use is levitation (both to toss Ace into the cafeteria’s chandelier and to evacuate campus visitors during Halloween), color-change magic (that he seems to struggle with), fireworks and ice magic.
He also mentions hitting targets with fireballs during a lesson.
Twisted Wonderland Reference Note 