The fastest way I've found to farm the credits at level 90 is in Zul'Gurub, go to the hut with the Master Chef and go to the top of the building. Start aoe'ing the neutral villagers and refugees, they instantly respawn when killed. I farmed 300 in less than 10 mins as an elemental shaman. I just set got an auto clicker to contantly press my chain lightning and went afk for an hour and i had enough to buy everything.