Congrats Drakengard!
Though truth be told, I don't think Luck or skill is that hard to get anymore, provided you are willing to spend time just to ruin your glicko as preparation.
I have been trying to get "impenetrable fortress" myself, but whenever I get a decent row, some hotshot comes by and ruins it.

20 successful defenses between my losses on Jun 1, 2009 and on Sept 10, 2009 (Hi Lucy!).
17 successful defenses between my losses on Sept 10, 2009 and on Nov. 28, 2009.
22 successful defenses between my losses on Nov. 28, 2009 and on February 22, 2010.
I know of only two dragons who have it, though there might be more. The easiest way to get it, as shown by one of these dragons is probably to get a lot of losses before you grow strong and rich, then those losses can camouflage how tough you have actually become, so you keep getting easy incomings.
In both cases, I had the honour of being the first one to penetrate the fortress after the achievement.
