the same arguement can be said for only pitting them on enemies that favors the griefmaker's armor penetration aswell, since there are endgame enemies with low enough armor that strength and gloves of giant strength negate their armor alone with their armor penetration, so testing on all enemies is a better way to test them, specially since you are still marching through areas with early game enemies aswell and seeing what finishes those fights the fastest is also importent so you dont waste time on the crap mobs
also if you have put everything in to strength then you have 17-19 strength and use gloves of giants strength, unless the enemy has an armor of 70 or higher (like 5 enemies i can think of off top of head) armor is a moot point as at the most the damage reduction will be 15- 17% (armor divided by 2) and considering the highest numbers i seen damage wise that like 8-14 points
The only endgame(so, Glacial Rift and Undermountain) enemies with armor of less than 35 (20 STR + 15 from the gloves) are the imps (3 armor), the Roc from the Avanai Rider encounter (17 armor), and Sleipnir from Liulfr(also 17 armor). There are a number of enemies with armor that's greater than 35 but less than 85, ranging from the 40's (Cultist Psion, Cultist Temptress, Selkies) and 50's (Batfolk Muggers, Glacial Rift Jotuns, Cultist Barghest), up to the 70's (Glacial Rift Siorcanna, Undermountain Kobolds, Undermountain Hobgoblins, Avanai Rider). After that, everything else (Liulfr, Earth Elemental, Mino Miners, Wraith Centaur, Cultist Centaur) is way up in the 100's, with the Mino Miners having the least at 92. Of these, the only ones with armor that can be overmatched by the Captain's Greatsword are clearly throwaway mobs - all the main threats tend to have at least decent armor.
I don't know why you guys are arguing over this, you can calculate the expected damage of each weapon pretty easily - so let's do it.
Against an enemy with (for now) 0 armor, 0 resistances, and 0 evasion, we will pit our Champion with 15 Accuracy (to counteract base miss chance), 0 attack power, 0 armor penetration, and 0 crit. We'll swing 100 times each, and assume the weapon will always do its base damage (there is some damage RNG ingame).
The Captain's Greatsword is expected to hit 95 times. We don't have enough Accuracy to get Accuracy crits, so of our 95 hits 19 of them will be upgraded to crits. Crits add 50 Attack Power, which in our case will always be +25 Penetrating Damage. Therefore, our expected average damage is (19*75 + 76*50)/100 = 52.25.
Galon's Griefmaker will hit all 100 times. It also doesn't have enough Accuracy to get Accuracy crits, so of the 100 hits 10 of them will crit. Our expected average damage is therefore (10*75 + 90*50)/100 = 52.5
As you can see, the Griefmaker is expected to do more damage
even without factoring in armor penetration. So why did Kyubi see more damage? That's because a while back, crits were bugged to do 50% more damage after all calculations. If that's still true, that means crits (which were supposed to only add 50 attack power) actually added a lot more, essentially 50 + 50% of your Champion's Attack Power as bonus attack power. Since low level enemies also have low evasion, he/she was conveniently sidestepping the Greatsword's main weaknesses: low armor penetration and worse accuracy. Now you might say wait - the difference in damage is mostly due to those 5 misses, so just having 5 more accuracy should even things out, right? Actually no. If we give the Champion exactly 5 more accuracy the Greatsword pulls ahead, yes, but more than that and the Griefmaker starts catching up, because it will get more Accuracy-based crits than the Greatsword. If you have more than 20 accuracy, 1 accuracy = 0.84 crit, more or less.
You also need to account for how Armor damage reduction works, which has a lot of finer points to it. 1 armor is 1% damage reduction, but with diminishing returns: 50 armor is about 33% damage reduction, and 100 armor is about 50% damage reduction. Against the Wraith Chimera, your base 35 armor pen is probably only getting you about 15-17 extra damage, but versus a Cultist Psion (41 Armor), that same 35 armor pen is probably getting you around 24-28 extra damage. The Griefmaker would only do about 5 extra damage vs the Psion, but should swing for around 27 more against the Chimera. That's only 143% more armor penetration, but about 180% more damage. In essence, each point of armor penetration is worth more than the last, so having low armor penetration is significantly worse than it seems. The highest solo damage build possible (Dual Wield Petal/Kunai), for example, rocks two Rings of Fortune for this reason.