Wihout perfect gear
The main problem about Bellower‘s gearing is that people are waiting to get the perfect Cruel/Offense sets. But it’s absolutely fine to skip the sets, because he can be excellent even without those perfect sets (or even without masteries).
You mostly need 2x C.Rate set (or even 3x) and some nice C.Rate substats anywhere else to reach a 100% C.Rate without gloves. Then you can complete with C.Damage gloves and amulet, ATK% chest and boots, and flat ATK on your ring and weapon.
The perfect gear
Perfect stats summary
You need at least 4.000 ATK and 230% C.Damage minimum, with 100% C.Rate. This is hard to achieve, but it’s worth the reward: getting a champion able to farm the Campaign in less than 10 seconds feels great!
The speed option
You can also go for a Speed option where you Bellower has enough speed to hit each wave twice before taking any damage. For that, in Brutal 12-3, you will need more than 203 SPD, with around 3.000 ATK and around 200% C.Damage, along with 100% C.Rate.