Based on my first night of farming, this was the quest seemed the biggest time sink of the Neutral ->Friendly, but your results may vary. (And if you're determined to be first on your block to be Exalted, you'll have to grit your teeth and power through it regardless.)
These crystals drop from essentially any mob on Netherwing Ridge. Above ground, you'll find nether rays lurking around the edges of the Ridge and flayers on the mountains further in. The flayers are more densely packed for easy farming, but that also makes them popular with your fellow grinders. The nether rays tend to be less over-farmed and the respawn rate is decent enough to keep a solo player from having too much downtime. Only risk is that some of the nether ray packs are near paths of the patrolling elite drakonids. These guys have ~100K health and hit hard, so best just to avoid them.
I found less competition in the underground mine, though that may change when more folks may find out about it. On the other hand, it's much less convenient to get to since you can't fly, so maybe folks won't bother.
You'll have to go fairly deep into the mine to find mobs. You'll see slimes and worms mostly in the central tunnel as you enter. Further in you'll get access to some side tunnels which feature flayers and ravagers. Way in the back you'll find murkbloods (hostile, unlike the friendly ones at the mine entrance). I was with a skinner, so we focused mostly on the flayers and ravagers and had decent results.
You can only turn in 40 Crystals per day, but you can collect as many as you can carry. Thus, you can get a jump on the next day's turn-in by farming longer.