In the Caverns of Aranaea after you enter keep going forward and follow the upper border and go northeast. If you hit a dead end restart, you should come across a connection hallway to different sections of the map.
On the bridge you should come across a purple spider named Arsect the Venomous. If you do not see a single purple spider(I have found that it only spawns here no other place, testing needed of course) then it did not spawn and you should restart the map.
If you do see the purple spider on the bridge then inside the cocoon next to it is a guaranteed goblin.
Purple mob and thus the cocoon and goblin inside is not 100% spawn, but the purple mob seems to be linked to that unique cocoon also. Needs some testing to find the spawn rate. But once you find the purple spider its easy to go to the cemetery and get 5 stacks and get goblin and repeat.
The checkpoint at the start of the caverns if your on quest Trailing the Coven should help for repeats.
Edit : Did some testing for spawn rate maybe I was just lucky initially and got a couple of spawns, but now I am not getting so much.
My apologies if spot sucks -_-.
Edit2 : Yep did more runs spot sucks sorry guys. The goblin and purp mob spawns there but rate is abysmal for me now.