I just came into this discussion, but heres what I think about bellum.
1. Doesn't have good army, just uses Mind Control alot. You know how in the Temple how when you kill a phantoms that this thing appears and that eye thing on his back kind of jumps than dies. I believe that those where the Ocean King's Men/ Knight's but Bellum gave new armour and weapon just like bellum gave a weapon and armour to Linebeck before you fought him.
2. Caution/ Trouble has his number 1 problem. Because you know how those eye things you see in the temple alerts the other phantoms. I think those were made by Bellum to make sure anyone doesn't get anywhere near him. Also I think when they alert the phantoms, Bellum gets the message as an enemy in the temple.
3. Bellum Fighting skills. I dont like his method of fighting. His first thing was when shooting those purple things at you. They than head towards you and they are easily killed. Middle Floor/Top Floor fighting. When fighting him when is linked to the pillars. He tries to slap you which is in fact the worst move ever when fighting someone.
4. Bellum Himself. Bellum's eye things is his weakness and he has too many around. He doesn't protect his main eye very much also. Also when Oshus was saying that bellum was great evil and etc. when describing him with his storyboard, bellum didn't show any great power of threat and evil, especially on the final battle.
Theres alot to talk about him but I narrowed it down to these 4 reasons why he is the worst boss ever fought. And I agree, Bane of Courage was tougher. Probably the Mind controling was what made him so strong.