I've been wandering around Draenor recently trying to get Grand Treasure Hunter and have re-discovered a mob I had trouble soloing when I was a fresh 100:
At the time, I had pulled the rare with every confidence that my BM hunter hax would pull me through (as it had for every other rare I'd encountered thus far), but even burning all my cooldowns, trying different pets and having a couple of random buffs wasn't quite enough to get him down.
With my heroic BRF gear now, I can now solo Echidna easily.
Extremely satisfying to come back and knock this guy over - it's not usually my style to admit defeat! Probably would have kept trying at the time too, except I had been under some pressure to gear up quick - most of my then-guild were already raid-ready on multiple toons and I didn't want to be *too* much of a drag on the rest of the raid.