Flappy Cat is the 1st stage of The Crazed Bird. Next stage: Forest Beasts.
Crazed Bird Cat has 1,180,000 HP, hits for 12,000 in an area, and reaches 640, further than anything a no-gacha box can hit from. You can't out-range it, and trying to kill it early just drains your wallet into its attacks. The stage is really about what comes with it: a Rain D. every minute or so and a Bore every four. Handle those cheaply and the Bird dies to chip damage along the way.
The lineup that does it
Ten slots, no gacha units, and a lot of Crazed cats. Crazed Macho Cat, Crazed Wall Cat and Eraser Cat stall the Bird and the Rain D.s. Crazed Whale Cat and Island Cat are the Bore answer. Macho Leg Cat is cheap ranged damage. Crazed Sexy Legs Cat, Crazed Dragon Cat and Crazed M. Titan Cat do the real work on the Rain D.s. Jamiera Cat rounds it out. Eraser Cat, Island Cat, Macho Leg Cat and Jamiera Cat are basic cats' True Forms, and here they need to be around 20+10, because their second forms aren't strong enough. If you're missing one, Ururun Cat from Stories of Legend fills the slot well. Her knockback hits the Rain D.s and the Bore, and putting both Legs cats on the top row with her makes her knockback proc more often. If you own Fisherman Cat, he adds a defense combo too.
Treasures and items
Every Empire of Cats treasure at Superior, plus Blood Fruit from Into the Future, because Crazed Whale Cat is strong against Red and the Bores are Red. The Alien crystals don't matter on this stage. Rich Cat isn't needed, because the opening below pays for itself. Speed Up shortens a long stage, but switch it off whenever two Rain D.s or a Bore are on the field, because double speed punishes every wasted unit. Sniper the Cat is a help, not a must: it pushes the Bird back, which gives your units more time on the Rain D.s. This clear managed without it.
Blood Fruiteffectiveness of Anti-Red abilitiesCrazed Whale Cat is strong against Red, and the Bores are Red 
Sky Fruiteffectiveness of Anti-Floating abilitiesonly with Ururun Cat, whose knockback also targets Floating SAFE TO SKIPAqua Crystal (no Aliens on this stage) · Plasma Crystal (no Aliens on this stage) · all treasures →
The opening: stall and upgrade
Send a Crazed Macho Cat straight away. Every time you can afford a Worker Cat level, buy it and send one Crazed Wall Cat, until the Worker Cat is level 6. That rhythm keeps the Bird stalled out in the field without spending on anything else. Then stop, because the first Rain D. is about to arrive.
one Crazed Macho out, the Bird far away · plays here, from that second
Rain D.: bring it home
Let the Rain D. walk all the way to your base, away from the Bird, and fight it there with Crazed Walls in front and Crazed Sexy Legs, Crazed Dragon and Crazed M. Titan behind. Those three matter because the Rain D. gets knocked back constantly, and each of them does two knockbacks' worth of damage per hit. Weaker units just push it back behind the Bird, where it lives longer. With one Rain D. on the field, that's all you send: Crazed Walls on repeat plus the three hitters. Use the quiet to max the Worker Cat.
the first Rain D. lured to your base · plays here, from that second
Bore: send everything, and the Whales
The first Bore comes at about 3:07, with another roughly every four minutes. When one shows up, send every meatshield, and both Whales without fail. The spare cash from the Rain D. phase pays for it. With the Whales going out constantly, the Bore goes down without much trouble. When two Rain D.s are on the field, add Eraser Cat to the Crazed Walls and the three hitters.
Crazed M. Titan on a Rain D., the Bird behind · plays here, from that second
Ignore the Bird
The Bird dies to chip damage while you're busy with everything else. Crazed M. Titan does most of it: it reaches the Bird while it fights the Rain D.s, and its Wave now and then hits the Bird too. Don't send units at the Bird. Deal with each Rain D. and each Bore as it comes, and the Bird eventually falls with money still in the bank. Then keep handling the Rain D.s and take the base. If you already have Awakened Bahamut from Into the Future, the stage is much easier. Cyberpunk Cat's Slow and range make it easy too, once you have it.
the chip at work, the Bird worn down · plays here, from that second
THE SHORT VERSIONStall the Bird with one Crazed Macho and a Crazed Wall per Worker Cat level up to 6, lure each Rain D. to your base and hit it with Crazed Sexy Legs, Crazed Dragon and Crazed M. Titan, send everything plus both Whales at each Bore, and never chase the Bird. It dies to chip.
The general approach, from the wiki's strategy notes. The guide above walks through one full clear.
A long fight: Crazed Bird Cat is hard to reach before Into the Future is done, and the Bores and Rain D.s cost you money and ground while you work on it.
- Crazed Fish Cat is a must: cheap, tanks Crazed Bird and handles Bore; Dancing Flasher does similar work and slows Bores; neither handles Rain D.
- Spammable ranged attackers (Macho Leg, Paris, Dragon) beat the Rain D.s; Witch Cat's crowd control on Bore stops you losing ground.
- Sniper the Cat damages Crazed Bird and wins ground back at the same time; Sadako stalls the boss but dies fast to Rain D.
- Cyberpunk Cat, once you have it, trivialises the stage with Slow and its range.
This enemy occasionally fires off a devastating attack at extreme range in a very fast animation and has a lot of health with just one knockback. Fortunately, however, it's fairly slow for the threat it poses, and only really matters as a support unit when other, more dangerous threats enter play.
CRAZED MACHO CAT A Stall in the guide's clear. 
CRAZED WALL CAT S Stall in the guide's clear. 
ERASER CAT S Wall in the guide's clear. 
CRAZED WHALE CAT A Vs Bore in the guide's clear. 
ISLAND CAT A Vs Bore in the guide's clear. 
MACHO LEG CAT A Ranged in the guide's clear. 
CRAZED SEXY LEGS CAT A Vs Rain D. in the guide's clear. 
CRAZED DRAGON CAT A Vs Rain D. in the guide's clear. 
CRAZED M. TITAN CAT B Vs Rain D. + chip in the guide's clear. 
JAMIERA CAT A Wall in the guide's clear. 
DANCING FLASHER CAT A Dancing Flasher Cat - Sporting high stats for the cost and powerful attacks, it can help deal with Bore and Crazed Bird itself similarly to Crazed Fish, and has the capability to slow down Bores. Like CFish, however, he has low range. 
SADAKO CAT A Sadako Cat - Useful for both stalling the boss and allowing other units to take more hits, however its low range leads it to die from Rain D. very quickly. 
CYBERPUNK CAT A Cyberpunk Cat - At the point you're able to get this unit, this unit easily trivializes the stage with its Slow ability and insanely high range, can help knock back the Rain Ds, providing excellent crowd control. 
SEXY BATHTUB CAT A Spammable ranged attackers (such as Macho Leg Cat, Paris Cat and Dragon Cat) work best against the Rain Ds, and Sexy Bathtub Cat helps against everything but the boss. 
DRAGON CAT B Spammable ranged attackers (such as Macho Leg Cat, Paris Cat and Dragon Cat) work best against the Rain Ds, and Sexy Bathtub Cat helps against everything but the boss. 
PARIS CAT A Spammable ranged attackers (such as Macho Leg Cat, Paris Cat and Dragon Cat) work best against the Rain Ds, and Sexy Bathtub Cat helps against everything but the boss. 
AWAKENED BAHAMUT CAT S In the event that you have cleared Into the Future, just sending out one Awakened Bahamut Cat at the right time can already drain almost 50% of Crazed Bird Cat's health. ⚙ Trait counters worked out from the roster — the guide's plan doesn't need them; kept for reference
Crazed Bird Cat is Floating. Anti-Floating units take less damage and deal more.
SABER THE CAT C Saber Alter Cat: Strong against Floating enemies (Deals 1.5x damage, only takes 1/2 damage) 
ARCHER CAT A Deals Massive Damage to Floating enemies. 
BODHISATTVA CAT A Slows and (True) Weakens Floating enemies. 
CATMAN S Massive Damage vs Floating (Normal). 
HIGH LORD BABEL S Equally tough against Floating enemies, with Colossus Slayer. Crazed Bird Cat reaches 640 range. Units that out-range it can chip safely without taking hits.
Crazed Bird Cat has a deep HP pool (1,180,000+ even before stage magnification). Heavy single-target burst shortens the fight.
BAHAMUT CAT S Huge multi-hit burst — a story-unlock powerhouse that deletes tanky enemies. 
TECOLUGA S A 7,300-AP long-range hit obliterates tanky targets when the Critical connects. Also fits “Out-range Crazed Bird Cat.” 
GAIA THE CREATOR S Long-Distance multi-hit with Insane Damage — a generalist wall-cracker. 
CRAZED MONEKO A Big crit burst against tanky/Metal targets. 
CHILD OF DESTINY PHONO S Big Multi-Hit Massive-Damage bursts from safety. 
The castle you destroy to clear the stage.