Molten Core

Dev, mobs in MC are over level 60. At 50 you wouldn't be able to look at them without drawing aggro, let alone damage them.

Think on this:

They're labelled as 63 Elite, but it takes a full Raid of 60's 30 seconds to a minute to trash one.

Also, these usually have over 600 damage per second AoE attacks/etc. I had one crit me (using my shield at 4k defense) for 2100 or so.