As it currently is, the level template is like this:

Levels: Earth Defense Force 2025
Chapter 1 ReconveneSpreading DisasterWinged War GoddessSetting SunWind BreakUnderground CaveEruptionRetaliationBeach FearThe Return
Chapter 2 Air ForceDay of the DemonsTakedownPreventing LandfallLand CollapseFortress SiegeValley ShadowsArtilleryBreaking ThroughShining Stronghold
Chapter 3 Into the DepthsA TrapGiant RobotsAnchoredSilver CityCrimsonHigh SpeedBrute ForceCharge Phase 1Charge Phase 2
Chapter 4 Charge Phase 3ThunderInfiltrationConvergenceInfernoStarshipDenseCrossingSearchWaterside Bugs
Chapter 5 AnnihilationNatural CalamitySteel BeastVoid ShipBlue FightersEarth DefendersMountain Pass TrapFortress FrenzyPincerRavages of War
Chapter 6 Air MobilizationBug InvasionTransport Ship Surprise AttackBug KingCrazy SkiesThe Fleet BelowBad PremonitionCity of HorrorMountains AblazeBreaking Through
Chapter 7 Bug TowerFortress AttackMountain LiberationSniper AttackTowering BeastTeeth of the UniverseGiant ShipSpearheadTower SiegeDen of Thieves
Chapter 8 Death QueenMachine SquadMonster PartyLabyrinth of LightCity FlawsMonster CampFallen FlagshipHeaven's Army'EncirclingDivine
End Game The ThreatBattle to the DeathMassive MobilizationScorchingStar Eaters

What I was thinking of doing was condensing this into just about 3 chapters

Levels 1 - 39 chapter 1 or something along the lines of "Ravager Return"

Levels 40 - 69 as chapter 2 or "The Earth Eater Threat" or "The New Threat"

Levels 70 - 85 as chapter 3 or "End Game"