I just did it - took me 2 tries, and this particular challenge always gave me trouble before. I looked at the vid, and I can't even say what was different. Goblin.
And I'm not sure you can do it with non-goblins except possibly halflings. That is, without preps. With preps - different story. And you might have a point about gold challenges becoming available before players are ready for them, it's been brought up before but must've got lost in all the stuff that's being done. It's still a beta, you know.
With preps, I'm pretty sure I've done it with a non-goblin and in a different manner.
EDIT: But gold challenges are different from regular gameplay - they're more puzzles than dungeon runs, and since most of them don't cost a lot they sort of require you to figure out optimal solutions. They're nintendo-hard so to speak, most of them. I'm really curious about what you've been doing that you found boring, and I'm also curious how you managed to not unlock the goblins after 12 hours of play. I generaly unlock them in about an hour without even trying.
I almost got pwned by Shifty Brickwork!