For all you Dark Knights out there

Well that makes sense, Achilles is a permanent defense, therefore it would decrease the number of hits needed overall. Aura wouldn't do that because it heals you over time, so if skelegons did enough damage to kill you in one hit Achilles would reduce that damage before it hit you. Aura would heal you AFTER taking the hit, so you'd already be dead by then.

It'd be very risky to be training at Skeles if they 1HKO you without Achilles.
At lower levels of Achilles, either you're going to underpot and die, or overpot. =/