Festival Gross is the 1st stage of The Crazed Gross. Next stage: Muscle Party.
Crazed Gross Cat is weak on paper: 120,000 HP, 100 range, speed 6. But every hit he lands sets off a Wave Attack, and five of them come out at the start, four more at 1:23, and then one every 21 seconds for the rest of the fight. A Wave hitting your front unit wipes out everything stacked behind it. So the usual plan of stacking meatshields and hitting from behind is exactly what loses. What wins is a clump of ranged attackers that knocks each Gross back before he can swing.
The lineup that does it
Macho Leg Cat RANGED 
Dragon Cat RANGED 
Crazed Dragon Cat RANGED 
Holy Valkyrie Cat RANGED + FREEZE 
Jamiera Cat MELEE 
Island Cat MELEE 
Crazed Whale Cat MELEE 
Crazed M. Titan Cat MELEE Eight units in two groups, with no gacha units. The ranged group: Macho Leg Cat, Dragon Cat, Crazed Dragon Cat and Holy Valkyrie. They hit a Gross from well outside his 100 range, and Holy Valkyrie's Freeze buys time when it procs. The melee group: Jamiera Cat, Island Cat, Crazed Whale Cat and Crazed Mythical Titan Cat. They're the tanky units that finish Grosses that get too close. Macho Leg Cat, Jamiera Cat and Island Cat are basic cats' True Forms, and here they need to be around 20+10: at their lower forms they barely matter on this stage. If you own Fisherman Cat, put him on the top row next to the Whales. That makes two Cat Combos, and the unit-defense one keeps your clump alive longer.
Treasures and items
Every Empire of Cats treasure at Superior, and Mysterious Force from Into the Future for a faster cat cannon. Cosmic Energy helps too. None of the enemies here have a trait, so the fruit and crystal treasures don't matter on this stage. Bring Rich Cat, because the Grosses start coming immediately and this stage eats money. Bring Sniper the Cat, because knocking Grosses back is the whole plan. Cat CPU can finish the run for you once every unit is going out anyway.
Mysterious ForceCat Cannon recharge time −5 seconds per chaptermore cat cannons, and each one knocks the Grosses back 
Cosmic EnergyCat Cannon attack power +500 per chaptera harder cannon SAFE TO SKIPSky Fruit (no Floating enemies here) · all treasures →
Ranged first, and never mixed
Let Sniper the Cat handle the first Snaches while you bank money. Then send only the ranged group, with Jamiera Cat just behind it. The rule for the whole start: never send the ranged group while a melee unit is being hit. A Wave hitting that melee unit also hits every ranged unit walking up behind it, and you've paid for nothing. When melee units are fighting, let them finish or die, then send the ranged group.
the ranged group only, the first Grosses arriving · plays here, from that second
The clump decides everything
The stage works like a snowball, either way. If your ranged clump kills the first Grosses cleanly, it grows, the next Gross gets knocked back before he swings, and the clump grows again. If a Gross lands a few hits, the clump thins, two Grosses end up together, and every one after that is worse. So judge the run in its first minute or two: if the Grosses start bunching up and your clump isn't growing, force-close and try again. It's a no-continue stage, and a restart costs less than a lost run.
Holy Valkyrie out, the Grosses starting to bunch up · plays here, from that second
Then everything, and push
Once the clump is thick and has passed the middle of the field, send everything, melee group included, and keep pushing. Fire the cannon whenever you want the whole line hitting the same Gross at the same time. Reaching the base isn't the finish: Grosses keep coming, and a weak push still gets knocked back.
the push onto the base · plays here, from that second
With gacha units
A Wave Blocker such as Octopus Cat (Kotatsu Cat's True Form) makes the stage almost free, since the Waves are the only real threat. Salon Cat is the other big one: a Rare you can get from the capsule, 350 range and area damage. Spam her as Paris Cat, or better as Cyborg Cat at level 30 and up, with Macho Leg Cat and Dragon Cat, bring no meatshields, and she keeps the Grosses back on her own. Keep one Titan Cat as an emergency block if a Gross gets close. Awakened Bahamut (after Into the Future Ch.3) can knock them back endlessly if your timing is good. Hacker Cat is a trap here: the line moves back and forth too much for him to land shots.
THE SHORT VERSIONNo meatshield wall: send the ranged group first and never behind a melee unit that's being hit, grow the clump until each Gross is knocked back before he swings, and force-close any run where the Grosses start bunching up. Full treasures, Rich Cat and Sniper.
The general approach, from the wiki's strategy notes. The guide above walks through one full clear.
Usually your first Wave Attack stage: every Crazed Gross hit on a meatshield triggers a Wave that decimates the stack behind it.
- Bring no meatshields; stack ranged attackers from the start so the Grosses are knocked back before they land a hit.
- A Wave Blocker such as Octopus Cat makes the stage trivial, since the Waves are the only hard part.
- Awakened Bahamut knocks the Grosses back endlessly (needs Into the Future Chapter 3 cleared); Brainwashed UFO Cat's range and Wave immunity do the same.
MACHO LEG CAT A Ranged in the guide's clear. 
DRAGON CAT B Ranged in the guide's clear. 
CRAZED DRAGON CAT A Ranged in the guide's clear. HOLY VALKYRIE CAT A Ranged + freeze in the guide's clear. JAMIERA CAT A Melee in the guide's clear. ISLAND CAT A Melee in the guide's clear. 
CRAZED WHALE CAT A Melee in the guide's clear. 
CRAZED M. TITAN CAT B Melee in the guide's clear. 
OCTOPUS CAT A If one has a unit with the Wave Blocker ability, such as Octopus Cat, the stage becomes trivial, since the only difficult part of the stage are the waves produced by the Crazed Gross Cats. 
BAHAMUT CAT A If you have Awakened Bahamut, this stage also becomes a joke, as he'll endlessly knockback the Crazed Gross Cats before they can attack. 
BRAINWASHED UFO CAT A Also Brainwashed UFO Cat makes this stage a cakewalk, due to its high range and Wave Immunity. 
The castle you destroy to clear the stage.