Sir Metal Seal has medium movement speed, but a faster attack rate than Sir Seal, higher attack power (close range and hits multiple cats) and very high health for a Metal enemy (2.75 times that of Super Metal Hippoe). Like all other Metal enemies, Sir Metal Seal only takes 1 damage from all attacks except for Critical Hits.
Do not use Bronze Cat against Sir Metal Seal. It has the same range as Sir Metal Seal, and a longer attack animation, so it will most likely get hit first and get killed. You may get lucky, but even then, it's only a 20% chance to knock back Sir Metal Seal.