Hmm, then I really cant see how the skill of the pit wrath lord or other +% damage skills can compete with the ability of the brim smith when it comes to dealing damage based on the attack stat. They are useful for other purposes ofc, but for an attack based build it is much more important that you get your attack stat above the enemies defence rather than doing more damage when you get above it.
Quick comparison assuming 3000 base attack, 1000 artifact attack and everything is gone (50% health).
brim smith in battle: 6000 atk, does respectable damage to enemies with 4500 defence. (6000-4500 = 1500)
pit wrath lord in battle: 4000 attack, does almost no damage to enemies with 4000 defence. ((4000-4000) * 3.5 = 0)
So unless your artifacts make up a very low amount of your attack stat, the brim smith ability is the way to go. I would even say it is one of the best late game abilities when it comes to dealing with the hardest floors you can manage assuming that you focus on the attack stat ofc. Even more so if you combine him with a brownie captain or something else that boosts total attack (Im I right in thinking that such a boost from a brownie captain would add to 6000 * 1.3 = 7800 in my example above?)