For all you Dark Knights out there

For bosses, yes, I could see why achilles is better, but think about training. L25 achilles is 12.5% damage reduction. On skeles, they do 3300 damage, that's saving about 410 HP a hit. It's easy enough to only get hit every 4 seconds, since if you're getting hit more than that, you're probably taking the smaller touch damage, so 500 HP > 410 HP.