In order to do this, there are 3 requirements needed:
1.) You must corrupt your shield ability:

2.) And then you need 2 perks:
"Shield Overdrive" - which will make shield regeneration much much faster
"Shield Converter" - which drops your health to 1 but will convert some of the rest into the shield

3.) The 3rd and the most important one is ARMOR. The more armor you have, the more your shield can last. The rule here is that the Damage Taken is less than the Shield Regeneration.

This is my first guide, please go easy on me, now for the main topic. You only need to go to the second Region and go to the bottom right corner of the map, somewhere in that location, you will encounter an endless horde section.
There are 3 ways to get into this location, the first one requires you to have the Dash ability. You can get this on the 2nd region while just passing through the shrines. The second way is to Corrupt the Double Jump ability which then adds a feature which will let you fly for a few seconds. And the last needs the Double Jump ability, but in order to get to the 2nd region you need to have this ability, so basically this is not really a requirement since you already have this by the time you get to this location. All you have to do is to damage a little hp to jump over the spikes and then keep hopping on the wall until you get to the top.
Then just keep on moving until you get to the rightmost part of that place, once you get into the corner, just stand in there and let it happen.

So what is happening here, basically the horde will swarm you from one direction, and the exploding shield will trigger, those minions will die one by one, but since it is an endless horde, they will just continue coming in, therefore unlimited profit.

Okay for starters, that is the only endless horde in the whole game (unless I forgot something) that is composed of melee creatures that would swarm you. Of course, there will be other hordes that would provide you higher chances of shards especially in the 3rd Region, but most of the hordes there were known to use ranged attacks, therefore you can't kill them unless you go in front of them and hit them, then that wouldn't be AFK farming. With this location, these melee creatures will swarm you, and the exploding shield will trigger. Another thing, this location provides a corner, therefore, swarms will come in one direction, and you won't get pushed and pulled into danger while you're in AFK. That corner is also special since there is an upward slope right in front of you and is also curved. As you can see in the picture:
Therefore, that will basically be your catch basin for shards.

Take note that I did this just after I finished the first boss. While playing, I realized that I need to study for my engineering exam and due to the rush, I just managed to alt tab while working on my PC and after some time, I went back only to see that I already died but gained 50K. Then I used up all my shards to go back again. The first one lasted for around 10 minutes, with 50K shards. Then my second run lasted around 30 mins before I died with 100K+ shards. After those 2 runs, when I tried the 3rd run, I observed that the shield regeneration now surpasses the damaged dealt by the creatures, therefore the shield would last forever. After that, I continued studying for around 2 hours only to get back that I managed to unlock the 500K achievement. Also, by doing that, I managed to unlock all the skill tree and the game became easy until the final boss.

I tried playing where you destroy the shards, and while not being AFK, this area is still good for farming shards on when you are still a low level character.