They only come out at night - and only in one place - a town park - and getting five (x5) Midnight Bride Herbs? Yeah, that just might be the toughest mission you have done so far!
So here is the thing - you have spent some time at night - but only a breif amount. This is not like that. This is moving through the full night while trying very hard to avoid the patrolling Volatiles. Why? Because chances are if a Volatile gets to you you are already dead.
That being so, the trick here is to avoid fighting with them. Select the closest Safe Zone to the park and to there, rest until night, and then Ninja your way to the park and collect the five herbs, then Ninja your way back to the Safe Zone.
That may sound simple, but it is not. So, hear this in mind - you can use Firecrackers to steer the Volatiles away from you - if you mess up though and one gets on your trail you can use the UV light or UV flares to stun them long enough for you to break line-of-sight and HIDE.
Once they resume their random patrol pattern you can continue to sneak towards whichever destination you are headed
If things go really bad though, and it looks like you will be overrun, then use some of Zaid's Flares because they will function as a portable Safe Zone. It may also help you to make and use some Night Vision Booster to help you see better.
Once you collect the fifth sample that part of the quest is done - don't hang around looking for more because there IS no more. Head back to the Safe Zone immediately. Slowly. Ninja-like. Once you make it back to the Zone, rest through the night and then head back to the marker to turn in the herbs and collect your reward.
Well done you! You completed a pretty tough mission - and if you completed it without dying and without getting into any fights - like we did - you did very well indeed!