To earn this trophy, you must kill 20 enemies (zombies of all types/ humans, etc.) in a row without taking damage of any kind. Not only does this mean avoiding melee/ ranged attacks, but also includes fall damage, electric shock, fire, splash from explosions, etc. You can not lose any health for any reason.
Although it sounds like a decent challenge, if done properly, this trophy is very easy to acquire. Do not try to complete this with melee attacks, because you will fail. The simplest way to earn this is by finding a heavily populated area of basic zombies, preferably where they are grouped up in several packs of 5 or more. (If you are having trouble finding such a place, I recommend checking your map for unsecured Safe Zones, designated by a red house icon).
For this method, you'll be using Molotovs. You'll earn the blueprint needed to craft these upon completing the first main mission from Spike, very early on in the story. After which, you should have the necessary supplies ( 1 Alcohol, 1 String ) to craft them. (If not, you will find these frequently from everyday scavenging). Each crafting will yield 3 Molotovs. I recommend crafting 2-3 sets, arming you with 6-9 Molotovs. This will allow you some leeway in case you can't find large groups, or have trouble hitting them.
Finally, simply toss a Molotov (select them by cycling
, press
to throw) into the center of a zombie pack, or by attracting many zombies nearby, and throwing one in their path to you. All zombies hit will burst into flames, and any zombie who touches the flames will burn as well. Just keep enough distance so you are not accidentally hit by a charging zombie, or take any splash damage from your own Molotov. 4-5 throws should be more than enough to down 20 enemies. You can also use Throwing Stars to quickly pick off single stray zombies from afar without the need to waste a whole Molotov.
This trophy should come very quickly using this method. Feel free to offer any comments or tips, ask me for more details, or even add me on PSN (username Danokitty) if you need additional help. Check out my other trophy guides for Dying Light, and happy hunting!
-Dan