Lord of the Abyss is the last stage in Shadow Cosmopolis.
The battle will start with a 200% buffed Bore, along with some 2800% buffed peons spawning. When the base is hit, Boraphim will appear as a boss, along with Gabriels, Heavenly Hippoes, Mr. Angels, and an Angelic Sleipnir.
Once you begin, lure Bore near the base, then start spamming meatshields, Crazed Fish Cat, Island Cat, and The Flying Cat. However, don't spawn too many units, because you will need money saved up later. Once you kill Bore,spawn a few more meatshields, Crazed UFO Cats, and Flying Cats. When you hit the base, the main problem appears: Boraphim. You should have all money saved up, so spawn Ururun Cat, Holy Valkyrie Cat and Awakened Bahamut Cat. Spam meatshields, Crazed UFO Cat, Flying Cat and other units.
Angelic Sleipnir isn't much of a threat, so don't worry much about him. Just continue spamming meatshields, Crazed UFO, and Flying Cat.
If Holy Valkyrie dies, you might have a slight problem, but if Boraphim dies, congratulations! Now go and claim your victory!
Lineup: Crazed Tank Cat (20, True Form recommended), Eraser Cat (20+30), *Ramen Cat (36), *Michaelangelo Cat (no minimum level), Pirate Cat (True Form recommended), Onmyoji Cat (True Form recommended). Fill the remaining slots with damage dealers that have over 390 range, which you'll need to kill the Sleipnirs. An anti-Angel Uber Rare is recommended. *They're for the Art History Combo to make Ramen Cat even beefier against Angels. Rich Cat and Sniper the Cat are needed.
When you enter the battle, spawn Onmyoji Cat and spam meatshields except Ramen. Then, keep spawning Onmyojis, Pirates and heavy hitters. Keep doing that. When the Bore is knocked back, start spamming Ramen and deploy your anti-Angel Uber (if included).
When Boraphim appears, don't afraid, just spam everything you've got. The Boraphim will be weakened by Onmyoji while Ramen Cat tanks a lot of hits from him. You will see Boraphim getting knocked back by the long-ranged heavy hitters, and soon he'll be dead.
If you have Momotaro this stage becomes very easy.
Save up money for iCat. When Bore spawns, spawn iCats to permafreeze him. When permafrozen, stop spamming meatshields and iCat, and start spamming Momotaro, but just putting out a meatshield occasionally. When the Bore gets knocked back, slow the spam of Momotaro and start spawning Jizo (or some other support unit). When the Bore dies, start spamming meatshields and spam Jizo and occasionally spawn Momotaro. When Boraphim comes out, he should be permafrozen and you are free to kill him. Once he dies, spawn your units to kill the Sleipnir, and then you will win.