CTO Seal is a well-rounded enemy with a fast attack rate, long range and a 100% chance of freezing cats with each hit, which is pretty much permafreeze considering his freeze duration is more than 4 times his attack rate. His attack power isn't too high, but he is usually accompanied by enemies with high pushing power, which help him gradually move forward and eventually reach your back line if you're not careful.
Similar to Selena, the most reliable way to defeat this enemy is to steadily chip away at him with ranged attackers, all the while sending out lots of meatshields to ensure that he doesn't push too much. Prisoner Cat in particular is a good choice, as he has great range and is strong against Red. Gross Cat and Salon Cat are both outranged by CTO Seal, so avoid using them.