Utopia is Over There is the second stage in Sub-chapter 20, Silk Road.
A very tough level if you do not have Critical Hitting Cats. It is advised to use at least 4 meatshields.
During the Battle
The Boss Appears
Once the Boss has appeared, have as many critical hitting cats out, as Sir Metal Seal is a very tough enemy and often takes a lot of Critcial Hits to take out. The enemy base will continously spawn Super Metal Hippoes, so take care of some of the enemies quickly so you won't be swarmed. Have a Shockwave Cat like Crazed Gross to help take out the Hippoes hiding behind the Metal enemies as well. Having Space Cat and Crazed Whale in your lineup is a must. It is also advised to get Fish Cat's True Form, Island Cat, before trying this stage. If you happen to have Marauder Cat, you'll have a much easier time.
Cat Line-up: Island Cat, C. Whale, Space, Rope Jump (Optional but very helpful)
Start the level. The first few Hippoes have slow movement, so there is time to stall. It doesn't get bad until the Heavenly Hippoes spawn.
Spam Space Cat, Island, and C. Whale, along with meatshields. There is plenty of time to get Space Cats and other critical cats.
Once the boss is spawned, do not stop spamming. If your meatshields are dying quickly, don't spawn too many expensive units. Super Metal Hippoe moves much faster, so let him get close to the base, then spam Island/C. Whale. Hopefully, he should be defeated. Then keep trying to beat Sir Metal Seal, and win the level!
Lineup: Space Cat, Catasaurus, Island Cat, 2-3 meatshields, stackable cats such as paris, and any metal killer ubers you might have.
Just spam your critters and stackable cats while protecting them with meatshields. This is a very easy stage with critical hitters. You might not even need island, he only has a 2% crit chance.
Note: Using Riceball as a meatshield may help against the Super Metal Hippoe