I haven't worked with Cas enzymes, but I have worked with a large number of other enzymes. In almost every case, they have retained activity through many freeze-thaw cycles, as long as the freezing and storage were done properly. The protein stock solution should be divided into aliquots of no more than 100 microliters in microcentrifuge tubes. The aliquots should be flash-frozen to prevent the solutes from separating from the solvent. This can be done by immersing the tube in liquid nitrogen, or a dry ice-ethanol bath, or powdered dry ice. The frozen aliquot should be stored at -80 to -70 degrees or in liquid nitrogen. The aliquot should be thawed gently in ice or with hand warmth. It should be mixed gently before use, being careful not to agitate it so much that bubbles form. Inclusion of 10% glycerol in the enzyme storage buffer helps the aliquot thaw more quickly and is also protective.