The fastest endgame way to farm organic resources is with a
greenhouse in your outpost and the
Eternal Harvest power.
To unlock the greenhouse you will need
Botany Skill I and you need to scan a species of flora local to that planet to 100%.
The
Eternal Harvest power is found in the
Lambda Temple. The temple location is random for every playthrough, but you can found new temples by talking to
Vladimir at the Eye or following the
Power from Beyond quests.
Now simple enter in the greenhouse, use your power and harvast all plants (there are about 25 plants only in the ground floor). Rinse and repeat until you get 500 organic resources.
You will get the achievement in about 15 minutes.
Thanks to
samsaBEAR for an alternative without the need of build a greenhouse:
To add onto this, if you don't want to put the time/points into building a Greenhouse, you can go to the farm used in the Deputized sidequest (Cheyenne - Montara Luna - Waggoner Farm) and farm using their crops.
From where you land you'll see a barn, just to the right of that is a large circular object. Around it's base are 21 Metamaize plants that you can harvest. The circle is split into four rows of crops, I found I had to pop Eternal Harvest after every two rows as the range on it isn't quite enough to cover the whole circle. Pop a Quantum Essence and just run around collecting, took about 10-15 minutes.
tl;dr
Create a greenhouse in an outpost, enter it, use the Eternal Harvest power, collect all plants on the ground floor, rinse and repeat.