I've just built Fellhound, 5(*) seconds farmer. 

Fellhound farms 1 second faster than Bellower just because he runs out faster than the fat guy. Everything about Fellhound is similar to Bellower, I even borrow 3 gears Helmet, Shield and Gauntlet from my Bellower for him. 

Fellhound building includes easy and hard parts. 

- Easy: since he is 1-hit-kill-all, he does not need speed, 130+ is enough. If your void great hall is good enough, he does not need mastery as well.

- Crit rate must be 100+%, stage 12-3 is easier than stage 12-6. To farm 12-3 must be Void or Spirit, to farm 12-6 must be Void or Magic to avoid weak hits. 

- Hard: Bellower attack is [ATK] I use gears with atk%, Fellhound attack is [ATK][DEF], I use gears with atk% + def%. Crit Damage must be very high, 200+% or more. Bellower build is easier than Fellhound because he always attack all, Fellhound only has 1 attack skill, if that attack can't kill all, he then buffs around and cost up to 22 seconds.

Also, you must have very good void Great Hall, if you don't, you may have to add all Attack Mastery for him.