If you do not have some Nulgath-related pets like what you stated, then the only option to farm Dark Crystal Shards in 100% manner is through Essence of Defeat Reagent.
1. Put the item you want to stack in the bank before you complete the quest/kill for drop.
2. Turn in quest/kill until item dropped.
3. Put item back in the inventory.
4. Accept drop.
5. Repeat Step 1-4 until it's unnecessary.
I will assume that you use a trainer here, specifically Grimoire 3.3
1. Get all the necessary items for Merging.
2. Open Shop
3. Hold Alt
4. Press Buy (While still holding Alt) then press F4 on keyboard after 0.5 secs after pressing Buy. (The goal here is to press Alt + F4 before it finishes merging)
5. Re-login to check if the item you want is in inventory.
6. If not, Repeat Step 2-6 but add a little more delay (like try pressing F4 after 0.7 perhaps)
You can search the whole AQW section of mpgh for this bots (sorry i am lazy), and i can make one for you, i can also teach you (and I am willing! ) how to make one.