Ambient killable animals like pigs and (regular) tiny rats that populate most maps.

They predominantly spawn in maps with urban tile sets. A little testing showed that there are probably none on Athel Yenlui and only very few on War Camp. But there are some on Righteous Stand, Halescourge, Horn of Magnus or Into the Nest - and lots on Screaming Bell.

This map offers 18 different but fixed spawn locations for critters (see below); for a total of 22 critters. This should be enough to complete the quest in a single run, some misses included, in roughly 10 minutes.
Screaming Bell is overall a very good map to farm quests: It is short and speeding-friendly. Art locations are always close to the main route because it's narrow and, finally, it guarantees 1 loot die because of the fixed Rat Ogre spawn (which is also a monster kill).

• Critters scatter depending on proximity of moving players, bots and enemies. All shooting in their general direction (or at enemies close by that then start to move) triggers them to run away. I assume they share the same threat/awareness mechanic like other ambient enemies.
  
• Critters run for a couple of units when triggered, then stop and wait in place. Then they run again and disappear into the ground.
  
• Critters don't scatter when approached while being invisible (affects RV, Huntsman, HM, Shade career skills).
  
• Critters can't be targeted or killed by auto-aim career skills (affects WS & Pyro career skills).
  
• Critters have 1 HP so every single damage tick will kill them.

1. Recruit difficulty for less hordes and overall enemies.

2. You should do it in a private game or with a full party to minimize interference (bots will screw you over lots, I tell you)

3. Keep your distance when close to a known spawn location; when possible wait out a horde before in order to avoid critters being pulled.

4. Always go for the critter kill first when there are enemies close to it.

5. Alternate your route: This is especially important for critters #8 and #9 since they are very close to the most common route people take and will most likely scatter. Underneath the first grimoire, take the alley all to the right and proceed.
Be careful around critters #11 and #12 (a juicy one) as the market is a very narrow area and they are triggered when you pull enemies from above from the walkway.

Critters always spawn at the following locations. If you can't find a certain critter, chances are you already triggered it. There are three juicy locations that always spawn two critters each. Make sure to get them.

Happy hunting.