EDIT: THIS IS NO LONGER OBTAINABLE. NOT SURE IF THEY'LL MAKE IT SO FOR NEXT YEARS EVENT, BUT AS OF NOW, THIS IS NO LONGER OBTAINABLE.
The easiest way, IMO, is to simply do dungeons. As of now, all enemies are scaled to your level, so it doesn't matter which dungeon you do. The I version of dungeons are much easier than II versions and beyond, and usually shorter.
Make sure you drink the witches brew before you start, so you can get the Skulls.
If you're a tank or healer, this will be much easier as queue times are minimal. DPS, I suggest learning to tank, or take on World Bosses (in pretty much every cave you find, though the tip below DOES apply to them, and takes much longer).
Another tip, people seem to think EVERY boss will drop a Skull, this isn't true. Seems to be a few minute cooldown, so you can expect to get about 3 per dungeon, maybe 4.
I played a few hours about 3 days a week and got this on Oct 26th, you can still get it, but it'll require a lot of grinding.