Similar to Portal, Entropy: Zero 2 has specific ranked challenges that unlock an achievement.

Mirage Clearance
Earn gold medals on all challenge maps.


There are 5 categories in 8 maps to get a gold medal on.

All challenges are locked on the 'Combine Elite' difficulty, aka Hard, attempting to change the difficulty or backtrack to an earlier map will instantly fail the challenge.

Quicksaving/loading is allowed during the challenges.

Least Damage
Get through the level taking as little damage as possible.

• Commanding your squad to advance in front of you during firefights, and keeping your head down in the APC will help reduce damage.
  
  
• Headshots instantly kill non-armored rebels.
  
  
• Damage to armor isn't counted as damage for the challenge.

Least Bullets
Get through the level firing as few bullets as possible.

• Explosives, Xen grenades, AR2 energy balls, and the crossbow do not count as bullets.
  
  
• If you have to use bullets, don't use the prototype gauss pistol or the shotgun, they fire multiple bullets/pellets at once.

Least Time
Get through the level as quickly as possible.

• There are a few shortcuts in some maps you can use to get well below the par time.

Least Kills
Get through the level killing as few enemies as possible.

• Enemies sucked up by Xen grenades will not be counted as kills.
  
  
• There's a detainment mechanic that can be used by kicking weapons out of the hands of rebels and pressing 'E' on them to get them to surrender. Beware, they might get up and try to rearm themselves if you or another Combine soldier are not nearby. Detained enemies can also be interacted with for ammo depending on the weapon they were carrying, or health if they are a medic.

Most Mass
Each map provides you with 9 Xen grenades, with the goal to suck up the heaviest objects from the level to fulfill the required score.

• Objects that are not part of the level geometry that have a high "weight" value and large enemies easily rack up points for this category.
  
  
• Some maps provide extra Xen grenades that can be used for the challenge.
  
  
• The amount of mass absorbed correlates to what enemies are spawned, in the interests of the challenge, this mean if the grenades sucks in enough mass, it will spawn 1 to 2 Antlion Guards, which then can be sucked in alongside the Antlions they spawn with another grenade for insane amounts of mass.

Least Damage
Damage cap: 20

Starting ammo is limited in the first map, so you'll have to rely on your squad to take the majority of the damage and aggro. Keep in mind they regenerate health slowly, and you can tell their health state depending on the color of their outline, so call them into cover if they're taking too much damage. Use the prototype gauss pistol's charge shot around corners as often as possible. Make sure to pickup an armor battery below the starting stairs to reduce health damage.


Least Bullets
Bullet limit: 25

There is a crate full of SLAMS and 2 extra grenades behind you and down the stairs from where the challenge begins. Once you clear the first office, you can let the rollermines take care of the rebels, You can even hold and kick them directly into a rebel for an instant-kill, they break if they take any damage from explosives though, so be careful.

Least Time
Par time: 1:00 minute

You'll have to rush through the rebels to meet the par time, although you should still pick up the armor batteries. Shotgunners are still a pain to get past, take them down if they are in your way. Fortunately the AI gets overwhelmed when confronted by multiple foes. Use automatic weapons if there's a bunch of rebels lined up in the hallways.


Least Kills
Kills permitted: 5

Shotgunners and AR2 equipped rebels will be your worst nightmare without armor. If you have the health to afford it, try to disarm them with a kick. Kick-boosted rollermines do not count as your kills.


Most Mass
Mass required: 5,000

There's not much to absorb in the level, and most of it is of low weight. I recommend to make a pile of objects in the last room on the bottom floor, and send your own squad into the blast zone when you toss the grenade to raise the chance of meeting the mass requirement to spawn an Antlion Guard, wait a moment until it summons Antlions, and then repeat. You can repeat this with your remaining Xen grenades until a Gonome spawns instead.

If you are still short on mass, the SLAM crate beneath the stairwell at the start of the level, and the grenade crate at the bottom floor at the end are worth 600 mass each.

Least Damage
Damage cap: 35

Fairly difficult, there's a lot of rebels to kill after opening the control room door. You should bunker down by the stairwell and let the majority of rebels come to you. If your whole squad dies, Aiden will call for reinforcements in the form of manhacks, which should help clear some of jumping ones out. Save often.

There's an alternate strategy to get this done easily, by going to the door your clone locks before interacting with the monitor to unlock it, you can skip visiting the control room entirely if you're fast enough. This may be patched in the future and is also viable for the Least Time challenge.


Least Bullets
Bullet limit: 40

Your SLAM is best saved for freeing Scar Zero (the unique Combine soldier on the top floor), to have another squadmate for the rebel ambush. AR2 headshots are lethal.

Least Time
Par time: 5:00 minutes

After opening the door in the control room, you can jump out of the windows and platform down the scaffolding to go right to the end goal. Make sure you have enough health.

Least Kills
Kills permitted: 12

Your squad will be doing most of the killing, they regenerate health slowly so as long as they don't die instantly from snipers or shotgunner rebels they should eventually whittle down all the rebels in the area.

You can avoid using a grenade for the first sniper by simply walking into the room beneath his perch, at which point he dismounts the rifle. Once you're in the control room, you can let your team kill everything from the safety of the stairwell, or rush to the exit via the Least Time method.


Most Mass
Mass required: 3,500

The mass requirement is just low enough that you can just get away with using Xen grenades in rooms full of medium sized props such as the poker table room, the loose crates and barrels in the warehouse, the tripmined room, and the kitchen accessed with a grenade in the vent.


Addendum: I overlooked an easier method to complete the challenge with a single Xen grenade. There's a pile of steel beams in the warehouse, each one of them is worth 2,000 mass, lodging a grenade in the middle of the pile should provide more than enough mass.



Thanks and credit to user "tone" for sharing this discovery!

Least Damage
Damage cap: 25

The RPG wielding rebels are your biggest threat in this map, memorizing their positions and sniping them will save you a lot of hassle. In addition, lowering your view in the APC will significantly reduce the amount of health damage you take from bullets, as long as you have armor.

Least Bullets
Bullet limit: 40

The APC's rockets don't count towards the challenge, just use em' and not the machine gun.

Least Time
Par time: 5:00 minutes

You can skip both powering the gate and the Pit Drone warehouse to save a lot of time. Take the right path and dismount the APC, you can jump through the windows on the right side of the building to get past the gate and simply sprint across the bridge to the warehouse. To the right of the warehouse are some Xen grenades, throwing one right by the fence they're found next to will get it out of the way, and you can sprint to the end of the map!

Least Kills
Kills permitted: 8

Utilizing the same skips for the Least Time challenges is far easier than attempting to disarm all the rebels or letting your Hunter ally kill them. Be wary of the RPG rebels on the boat across the bridge and snipe them if they prove troublesome.


Most Mass
Mass required: 14,000

This can easily be completed with a single Xen grenade! After clearing out the first building of rebels, Aiden will call for a dropship to bring reinforcements. The dropship container is worth 50,000 units of mass. Wind pressure created by the dropship may prove difficult in actually absorbing the container, but a well timed "use throw" will easily absorb it.

Least Damage
Damage cap: 10

Should be relatively free. There's no enemies with weapons and Pit Drones deal negligible damage when you're in the APC.

Least Bullets
Bullet limit: 20

Easy. Rockets recharge quickly, and Pit Drones won't be a hazard under the wheels of your APC.

Least Time
Par time: 1:00 minute

Par time is pretty tight on this level. You'll need to spam the APC's turbo whenever it's available, but if you get stuck on a turn the challenge is already over, so try to use it going down straight lines.

Least Kills
Kills permitted: 1

The Vortigaunt atop of the cliff will shoot open the crates containing Pit Drones, but he's slow to fire, and you can easily outmaneuver the ones in the way. Careful not to drive over the crates in the tunnel.


Most Mass
Mass required: 9,000

There is a second APC shortly before entering the tunnel on the opposite side of the valley. Absorbing it and your own APC will provide a lot of mass and Antlion Guards to continuously reabsorb. If they don't spawn, the two abandoned cars near the crates containing Pit Drones provide at least 1,000 mass units each.

Least Damage
Damage cap: 50

Similar to APC Assault and Lake Tuurngait, having armor to prevent health damage from gunfire while lowered in the APC will be of great use. There is a crate with an armor battery behind you when you begin the challenge. Drive slowly and use the zoom function to take out the RPG rebels before they see you.


Least Bullets
Bullet limit: 1

You'll have to stick to rockets and vehicular slaughter. If the rocket tracking is having trouble targeting RPG rebels, you can still use the crossbow to snipe em'.

Least Time
Par time: 1:45 minutes

Just like Lake Tuurngait, the par time cuts close to what is possible without accelerated back hopping. Just spam your APC's turbo as much as possible, and try to shoot any props that may get in the way and impair the speed of the APC. Rockets can be used to blow down the fences in the last stretch of the map.


Least Kills
Kills permitted: 5

Not too different from the Least Time strategy, just drive slower and carefully to avoid running over the rebels, Pit Drones, and baby Bullsquids.


Most Mass
Mass required: 10,000

Pretty easy to get with few Xen grenades. The logs dropped on you by rebels can be absorbed, each is worth 1,000 units of mass. In addition, there is a second APC below a white Aperture container hanging off the cliff-side, using a grenade to suck both it and the APC you were driving gives 6,000 mass units, and should spawn Antlion Guards to provide any remaining mass.

Least Damage
Damage cap: 25

Pit Drone projectiles do a gross amount of damage, but you can strafe to avoid them.

Pit Drone projectiles and Zombiegaunt area-of-effect attacks are the main source of pain for this challenge. Most of the Xen fauna will attack each other, and the area can be mostly cleared out if you use the Xen grenades to close the portals.

Least Bullets
Bullet limit: 20

The two turrets guarding the tram will help you clear out the Xentarium without spending a bullet, and are also good shields against Pit Drone projectiles.

Least Time
Par time: 5:00 minutes

The minecart takes about a minute to take you to your destination, make sure you've started the ride before 3:00 minutes in. Xen fauna will be so distracted by infighting, you can pretty much sprint through the whole area without resistance.

Least Kills
Kills permitted: 5

Once again, the two turrets provided at the start of the level do not count against the challenge, and provide an easy method to clear out the Pit Drones and baby Bullsquids, they do eventually run out of ammo though, so you should still rush through the level while the Xen fauna are infighting.


Most Mass
Mass required: 5,000

There are 3 boulders by the end of the level near the lift that provide an essential amount of mass needed for this challenge (1,000 each). Two of which block your path, and the other directly in front of the previous 2 by a skeleton. The final room before the end of the challenge gives around 500 mass on average. This map has a surplus of spare Xen grenades.

There's 1 grenade in a crawlspace next to the elevator.
2 on the desk in the observation room the elevator takes you to.
2 on the catwalk after you drop down from the observation room.
1 to the right of the minecart at the end of the ride.
1 is in front of boulders mentioned earlier.
The last one is on the hanging cargo pallet after you reach the top of the chasm.

Optimally, you should have at least 1,200 mass before boarding the minecart. I recommend absorbing the Gonomes and turrets/shelves next to the cart.

Least Damage
Damage cap: 25

Magnum and the crossbow are handy for sniping rebels before they can get a shot off. Headshots with the AR2 and MP5K instantly kill rebels, save on their ammo for the bunker.

Least Bullets
Bullet limit: 20

You can kill the first 3 rebels with one bullet by shooting the rebel as he's throwing a molotov on the junk pile. The first rebel armed with a crossbow you encounter can be disarmed easily and interacted with for more crossbow ammo. You can also crush the Temporal Crab down the middle path for more crossbow ammo. Use the crossbow for any armed enemies in your way, and utilize your squad and grenades for the assault on Judith's bunker.

Least Time
Par time: 5:00 minutes

The par time is pretty forgiving as long as you keep moving. You can rush past the automated turrets as they're setting up.


Least Kills
Kills permitted: 5

The most annoying part of this challenge is within the first section. Pickup the Xen grenade the turret is facing and save it for the first 3 rebels. Disarm the others and snipe the AR2 wielding jumper rebel and sprint past the others. The abundance of Xen grenades will be very handy for sending your foes on a one-way trip to Xen. Make sure to save some for the bunker assault and let your squad take most of the rebels on.


Most Mass
Mass required: 5,000

Another easy one. The health and armor chargers can be absorbed for 50,000 mass units each.

This map is a freebie! There is a spot behind the container truck next to the security building that neither rebels nor zombies can path to, then it's just a matter of waiting for the transport platform to arrive. This allows easy completion of the damage, bullets, and kills challenges.



Least Damage
Damage cap: 60

Least Bullets
Bullet limit: 300

Least Time
Par time: 10:00 minutes

As long as you rush into the building to start the platform's arrival and board it immediately upon arrival, the par time is more than enough to finish.

Least Kills
Kills permitted: 5

Be wary of the hopper mines, if they are placed by you they will count as your kills.


Most Mass
Mass required: 5,000

This can be completed with very few Xen grenades. Suck up the 3 green dumpsters outside the house, this should cause an Antlion Guardian to spawn. The health and armor chargers here will also provide additional mass.

• Unlike HL2 and the episodes, you can fire while zoomed in.
  
• Jumping on top of a Headcrab crushes it. This also works on Temporal Crabs.
  
• Bullsquids can heal by eating dead bodies, they briefly turn green to signify this.
  
• Vortigaunts don't have perfect tracking, sprinting while strafing will cause them to miss most of their shots.
  
• If a Xen grenade absorbs just a skeleton, it will spawn demons from Hell.
  
• Using Xen grenades during the final boss fight will bring strange bedfellows to your aid.
  
  Xentarium
  
  • You can clear out the entire Xentarium, but two Bullsquid eggs will periodically respawn.
    
    
  • Approaching the small Xen growth pictured will instantly crash your game for some reason.
  
  
  
  The Last Stand
  
  • If you are absolutely deranged you can meticulously kick every desk, barrel pallet and crate into a big pile to absorb with a single Xen grenade. This serves no purpose and will probably lag your game.
  
  
  
  
  Thanks for reading!
  If you discover any objects worth an abnormal amount of mass, or have any esoteric knowledge about a map you'd like to share, let me know!
  
  If Steam fails to load a video on this guide, here's a link to the playlist.