For all you Dark Knights out there

Aura of the Beholder is a must have for training, especially at lower levels, where you can barely take two hits from the monster you're training on. It takes off a lot of stress for maintaining your HP at Berserk level; tops off your HP so that you can get your maximum HP below your Berserk limit without carrying a tonne of different pots and removes the need for you to try to figure out which pot to use all the time. This may not sound like a significant advantage, but you will think differently if you spend 3+ hours training. Having to concentrate so much on your HP gets very tiresome and it is the main drawback of the Dark Knight class, in my opinion. Anything to help with that is a godsend, and I've found that Aura helped me a lot.

Achilles might be useful for bosses, but I honestly don't know. I'm level 156, and I fight Horned Tail almost every day. Achilles will probably be one of the last skills on my list to max.

I agree, Aura is amazing during training when maintaining Berserk. It lets me stick to only one pot type instead of having to hold, say, white pots just to hold the maximum threshold. I just think that Aura isn't as useful in Horntail since it doesn't heal fast enough to compete with potting, at least to my extent of knowledge in fighting it. Also considering that you'd be healed by a bishop about 90% of the time, Achilles is more useful since it prevents damage from being done rather than restoring it after it's been dealt with. But I guess it depends on what you do more. And yeah, I Horntail more than train.