The Alchemest Goblin that gives the testamony is only in two or three places.
Labyrinth of Onzozo
That's 5 places off the top of my head. There could even be more than I'm not remembering right now.
Can't really say which place would be the best place to go nowadays, since they might have messed with the spawn locations and amounts. The good news is that most dungeon mob respawns that used to take 16 minutes are now 5 minutes, so you don't have to wait around as much for repops.