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.
Strategies
This is a Metal enemy, so you will need some Critical Hitters like Island Cat, Jurassic Cat, Marauder Cat, Space Cat, Rope Jump Cat, or Neneko, or Sunny Neneko to deal with this enemy. Unlike most other Metal enemies, who will die after one Critical Hit, this enemy generally requires several Critical Hits to take down. Tesalan Pasalan is a great choice here because his Critical Hit can kill Sir Metal Seal in one hit.
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.