Arnice can fill up her Transformation Gauge in the lower left of her screen by landing consecutive attacks on enemies. Once filled, pressing L2 will allow her to take a more powerful form for a limited time and will give her access to powerful attacks. Her transformation will also affect specific stats of her currently summoned servans.
Arnice can have up to four different types of transformations. These are determined by the Transformation Factor or the combination of the Servans equipped in your current party/deck. Each Servan has a transformation factor value displayed under their model or portrait in the Deck Loadout screen or the Servans' individual status screens. A Servan's transformation factor value can be increased by giving them the same fetishes (summoning materials) as their class/ type.
If the total value of the transformation factor of the same color in a deck party exceeds 10, Arnice can transform into that color's form. The color of the highest transformation factor corresponds a specific form. Red for Demon, Blue for Phantom, Green for Rabbit, and Purple for Armor. The higher the total of transformation factors, the faster the transformation gauge will fill up. The servan in the leader slot of a party will have his/her transformation value doubled so let's say an Alraune in the leader slot with a TF value of 5 will single-handedly influence Arnice's transformation to Phantom Form. Another example is that if a leader demon in the party has a yellow TF factor of 3 (doubled in leader slot so 6), as long as it is supported by at least another or more servans with TF of the same color totaling 4 or above, Arnice's transformation will be altered to a Rabbit Form. If these conditions are not met, Arnice can still transform in her default form, the Demon Form.
Transformations are very helpful and can be life savers when you're in a pinch so take into consideration what servans will make up your party and affect Arnice's transformation.