I'm not a Zeph farmer. I usually finish zeph in 5-10 min depending on the difficulty, monster, and map, but 20+ character per hour? either your exaggerating, lying, or been lied to. Every char has different quest means different monsters, even if you have multiple accounts receiving zeph at the same time. You would still have to farm different monsters in different maps. Alot of farmers i know can do zeph efficiently which still takes time and effort, testing and retesting different jobs and builds. If its true they can do 20+ char per hour, why can't you? that means you can do 60 chars a day assuming you play 3 hours a day
Don't expect to gain the same amount against people who farm more than you. Now you will get even less