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. If you have an anti-red Uber, Sir Seal becomes much more manageable.
Came to the big city from the quiet seaside. Wanted more attention so dyed his body red.