Seahorse Siren is the fifth stage in Barking Bay.
Battleground
This level is just Sir Rel spam. In total, there are 15 waves of Sir Rels with 8 Sir Rels each. The waves spawn around 10, 40, 80, 120 and 160 seconds. After 66.7 seconds, an Oldhorn will come out, and after 100 seconds, Sir Rels independent of the huge waves will begin spawning in 10~20-second intervals.
Strategy
Strategy 1 (Cheese)
Lineup: Crazed Cat, Crazed Tank Cat, Crazed Axe Cat, Crazed Gross Cat, Manic Lion Cat, Awakened Bahamut Cat (Lvl 40 preferred), Metal Cat (Optional)
First 5 cats are there for combo, you also need 175K+ Base HP.
Cat Cannon: Iron Wall (Optional)
This strategy does require a bit of luck, but the procedure is extremely easy to pull off. Start by sending a Metal Cat to block the 1st Sir Rel (if you have it) and spawn Awakened Bahamut Cat when the 2nd Sir Rel starts hitting your base. After that, spam your meatshields and Manic Lion Cat. Try to summon Metal Cat, too. If done correctly, Awakened Bahamut should destroy the base while your meatshields prevent Sir Rels from hitting him.
Strategy 2
Lineup': Island Cat, Seafarer Cat (29+1), Brah Cats, Lollycat, Ultra Delinquent Cat (29+1), Manic Mohawk Cat (30), Manic Eraser Cat (40), Eraser Cat (20+25), Manic Macho Legs (30), Slapstick Cats (35)
First 5 cats are there for combo.
Cat Cannon: Breakerblast
Don't upgrade your Worker Cat at all during this battle whatsoever, it is not necessary and is a waste of money, so that's why you use Worker Cat start up combos such as Sunbathers and Lingerie Special. Anyways, once the battle starts, wait until Sir Rel spawns and then send out your meatshields, Manic Macho Legs and Slapstick Cats, don't forget that you also have Seafarer Cat during the battle, as he can be helpful during the intervals where Slapstick Cats and Manic Macho Legs aren't attacking. The first Sir Rel should die quickly, spawn Slapstick Cats and more Manic Macho Legs if you can, you should be advancing at a mild rate, spawn a Seafarer Cat as well if your Slapstick and Manic Macho Legs start to get knockbacked, I would recommend zooming out when your about in the middle of the battlefield, to see if Oldhorn has spawned yet, if he has don't immediately advance towards him with all you have, start only spawning out meatshields in order to save up for Ultra Delinquent Cat during this period, once you are starting to attack the Oldhorn, save up for another Ultra Delinquent Cat immediately, but this shouldn't be that much of an issue this time. Now Ultra Delinquent Cat can still be knockbacked and hit during this section by the Oldhorn advancing if Oldhorn doesn't get frozen, and since Ultra Delinquent Cat has 3 knockbacks, if Ultra Delinquent Cat has been knockbacked twice already and the Oldhorn is about to hit him, use Breakerblast. Once the second Ultra Delinquent Cat is off cooldown, send another out and they should get Oldhorn in almost a perma-freeze state, after that the Oldhorn will eventually die and you will win.