I made a spreadsheet to figure out which monsters are best to grind. For both XP and gold, I found that you can either grind the toughest monster that you can take out with a single crit (get that crit % up!) or use both red and green potions and grind Golden Reptus, until you can get to Djinns. After that, your stats change the efficiency a lot, but if your CRITDMG without potions is above 0, the last 2 djinns and the first 2 fire blobs are pretty good. By then you should be able to either rebirth or go for medals.