This difficulty, different from the others, its name is not describing how hard it is or the scenario we may meet in the heist. Its name one down is from the downed times allow before we go into custody
What is downed times?
The number of downed time directly related to player going into custody
By using doctor bag or being release from custody, your downed time will be reset to 0.
Reminders:

• Being downed by taser and cloaker's special attack does not count as downed time
  
• Having the skills nine life Aced allow you to have one more downed time before going into custody

In one down difficulty, cops will have different equipment (body armor) and we called them zeal team.
Enemy which has different body armor:

• Shields
  
• Normal cops (2 variations)
  
• Dozers
  
• Cloakers

Other enemy will remain the same
The zeal team have a lot more health and deals more damage, which make the heist very difficulty.
Even with the highest armor in game, it will just gone in a few shot
One variation is wearing grey armor and helm
The other one is wearing black armor with skull mask
Rather then the normal flat shield that fbi or swat team have, the shield that being used by zeal team is slightly larger and the edge of the shield will bent a bit inwards.
With the new body armor, they look very similar to the normal members.
Besides the appearence, they have the same attack behavior.

In one down difficulty, since all the enemy deals great amount of damage, there are not much difference in terms of threat between specials and normal cops. Besides dozer, medic and sniper.
Since enemies deals a lot of damage, no matter what build you have, you would not be able to stay at an open field for long. Therefore, we have to find opportunities to do the objective. In between assault waves, most of the cops will retreat which allows you to do objective / accross open field more easily.
Delaying assault waves
To earn more time for objective, delaying an assault wave is an option. By having a hostage, the assault will automatically delay by ten second, however, having more than one hostage will not give bonus effect and converted cops do not count as hostage.
Trading your teamates
It is common to be in custody on one down difficulty, in the worst case you may be the last one standing. To gain back your team's fighting ability, trading back your teammate is essential, trading negotiation can only occurs during non assault period.

One down difficulty demand a lot of technique from players, as well as a well combined skills set. Making sure you have a good build before going into such heist. One down difficulty also requires co-operations between players, poor communication won't do the job, discuss who will bring what and coordinate skill points to make sure that your team have enough supplies and tools for combat.
Right before a heist, we can gain access to pre-planning or assets, make full use on them to help.
Health, is the most important in other difficulty. You can still stay alive with your heavy armor and your health. However in one down difficulty, Armor is what you have to pay attention on, make sure you always have a cover to hide and regen your armor. Stay with your teammates and help each other.
Besides staying alive, eliminating enemy is also important, there are a certain amount of enemy spawn in each wave, killing more cops meaning there are less enemy remaining and your team has a less chance on being swamped by cops.
Do as much objective as you can. Trading your teamate, taking hostage do as much as you can.

Since enemy are dealing so much damage, dodge builds are more prefered. And for pure dodge build, we usually wear suit which only have 20 armor. Facing enemy who deals 200-ish damage per shot, how can we still survive?
As long as we still have armor, a single shot from enemy will not damage over the bound of our armor (except sniper).
For example:
Therefore, having at least some armor will allows you to survive one more shot. (one shot can be critical in one down difficulty as your armor and health will be gone in less than 10 shot)

Dodging is another major aspect of surviving. Having XX% dodge chance meaning we have XX% chance to dodge every single bullet. (Needless to say right?)
The highest base dodge chance that a player have while stationary is 65% and 75% while sprinting, 80% while ziplining. These are all based on RNG (your luck).
This included:

• 5% from wearing suit
  
• 45% from rogue perk deck
  
• 10% from skills (top tier Artful dodger)
  
• 5% from AI boost
  
• 10% / 15% from skills (Duck and cover Aced)

Higher detection risk level will result in lower dodge chance gained, down to 0%

Electrical brass knuckles
During the heist, you may encounter some situation where you have to fight few enemy by yourself, instead of quickly shooting all of them, you can stun them all using this melee weapon and slowly take them down. The stun effect will remain around 4 sec.
Besides, ammo preservation is important in one down difficulty. Other than killing the enemy, you can intimidate them and make the surrender, stun them before hand will make them more likely to surrender.
Concussion grenade
This grenade have the similar effect of the electrical brass knuckles, this however, is a grenade, it will do an area effect instead of a single enemy. This can even stun the whole field of cops.
In some situation such as inspiring your teammate in open field, moving to other places, doing objective, this throwable will surely come in handy.

Few thing we should avoid:

• Ignoring your armor
  
• Leaving your team
  
• Killing civilians
  
• Not communicating
  
• Having poor skill set (especially for deployables)

Stealthing in one down difficulty are usually same as stealthing in mayhem and deathwish difficulty.
The major difference is that after being spotted by guard, the guard will take few more shots to kill.
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