At the first time you meet Sir Seal in Greece, use Axe Cat combined with meatshields, Fish Cat and Lizard Cat. This way, Sir Seal will be defeated quickly and easily.
Some time later on in SoL, Sir Seal's strength magnification will fluctuate a lot and give him generally higher strength magnifications than the other enemies introduced in Empire of Cats (i.e. Upset Rock) so use strong anti reds such as Whale Cat/Island Cat.
Sir Seal is especially vulnerable to the Freeze effect because with one knockback he can't actually escape stunlock, and the slow-hitting but powerful characteristic of a typical Uber Rare unit comes in handy against Sir Seal too.
Came to the big city from the quiet seaside. Wanted more attention so dyed his body red.