While fluidic elastomer actuators (FEAs) have been widely adopted for soft roboticsapplications showing potential for capabilities not achievable by their rigid counterparts, they have limited stiffness due to their innate compliance and cannot alter stiffness independently of their configuration on their own. Existing methods of combining FEAs with other mechanisms to achieve stiffness tunability limit the flexibility of the actuators and necessitate additional control inputs, or require energy to maintain higher stiffness and need onboard electronics, and may not work in the presence of electromagnetic fields. To address these limitations, this work proposes the use of supercooled liquid (sodium acetate trihydrate solution) as the multifunctional working fluid of FEAs to control both position and stiffness; we hypothesize that such fluid would be capable of both inflating the elastomer chambers while in liquid state and rapidly solidifying and stiffening the actuators. We investigate the mechanical properties of crystallized SAT samples and ways that they can be reinforced. Additionally, we analyze the stiffness of FEAs that use the supercooled SAT solution as their working fluid, highlighting the high degree of stiffness change (increase by 6.56 times) induced by the crystallization of the solution. Furthermore, we demonstrate the multifunctional manipulation and stiffness change of an FEA with minimal control inputs using supercooled SAT solution as the working fluid. The proposed mechanism is a new way to combine the high manipulability of FEAs with stiffness tunability and opens up new possibilities in designing soft robots and manipulators.