Project Clean Earth: How to Beat Chapters 71, 72, and 73

To clear Chapters 71, 72, and 73 in Project Clean Earth, prioritize continuous area-of-effect (AoE) stagger and barrier defense over single-target burst damage, while moving in wide perimeter loops along the arena edge. These stages feature updated Backroom layouts with tighter obstacle spacing and rapid swarms that can quickly body-block your bot if you attempt to hold the center of the map.

Core Build and Skill Priorities

Surviving the high-density waves in this chapter tier requires preventing small mutant runners from clustering inside your bot's hurtbox. Pair a primary multi-target weapon with automated defensive skills that push back approaching mobs:

Energy Barrier / Shield Module: Absorbs stray projectiles from ranged mutants and prevents lethal damage if charging swarms clip your bot while maneuvering.
Rotary Blades or Saw Orbiters: Provides constant point-blank knockback, creating an automatic safety buffer around your bot.
Homing Missiles or Cluster Rockets: Automatically targets distant spitters and chips down elite enemies while you concentrate on movement.
Drone Support: Supplies tracking covering fire to thin incoming waves without requiring you to stop and manually aim.

When early level-up options appear, prioritize weapon radius increases and cooldown reduction over flat attack power. An expanded attack radius prevents early swarms from surrounding you before your build reaches its key evolutions.

The narrow corridors and obstacles in Chapters 71 through 73 make standing in the center fatal. Lingering in open ground exposes your bot to simultaneous attacks from all directions.

Use these movement rules to maintain control of the arena:

Sweep the perimeter: Move in broad oval patterns along the outer borders of the stage. This forces incoming mutants to trail behind you in a single column rather than encircling your flanks.
Watch corner traps: Never commit into a narrow corner or dead end unless your shield module is fully charged and your rotational blades are active to clear a path.
Breach through weak flanks: If a swarm blocks your perimeter route entirely, cut through the thinnest section of the opposing line. Trigger an active barrier or invulnerability window to absorb the breach, then immediately re-establish your outer loop on the other side.

Chapter-by-Chapter Strategy

Each stage presents a distinct enemy composition that requires slight adjustments to your in-run upgrades.

Stage	Primary Threat	Key Skill Focus	Movement Strategy
Chapter 71	High-speed runner swarms	Rotary Blades, Area Expansion	Wide perimeter loops; stay out of dead ends
Chapter 72	Tanky bruisers and acid spitters	Piercing weapons, Homing Missiles	Lateral strafing; cut through the thinnest pack edges
Chapter 73	Elite ambushes and sweeping boss attacks	Shield Barrier, Cooldown Reduction	Standoff kiting; bait boss telegraphs into open boundaries

Chapter 71: Fast Runner Control

Chapter 71 opens with erratic, high-speed insectoids and scrap runners. If you do not secure a multi-hit AoE skill within the first three level-ups, these units will chip your health down before the first elite spawns. Focus on circular knockback and movement speed passives early, then collect experience gems systematically to hit core weapon evolutions before minute five.

Chapter 72: Armored Bruisers and Ranged Spitters

Chapter 72 pairs high-health melee bruisers with back-line acid spitters. The bruisers absorb direct frontal fire, giving ranged enemies time to bombard you. Take piercing skills or homing missiles that bypass front-line tanks to hit the spitters, and continuously move laterally to dodge incoming acid telegraphs.

Chapter 73: Mixed Waves and Boss Compression

Chapter 73 alternates between massive mixed swarms and sudden elite spawns. The final boss features wide sweeping hitboxes that leave minimal safe floor space. Save your secondary active skills and defensive modules for this encounter. Maintain mid-range spacing—close enough for homing rockets and orbiters to connect, but far enough to clear wide sweep animations without burning emergency revives.

In-Run Passive Modifiers

Establish your defensive perimeter by wave three, then invest in passive modifiers that scale your operational uptime:

Cooldown Reduction: Shortens the interval between shield refreshes and heavy weapon volleys to eliminate defensive downtime.
Movement Speed: Allows you to outrun runner variants consistently without burning evasive abilities.
Magnet / Collection Range: Lets you safely gather upgrade chips and health drops from a distance without diving into enemy packs.

Avoid low-impact utility cards that rely on conditional triggers, such as perks that only activate at critical health thresholds. Unconditional baseline stats provide far more reliable survival across all three stages.