Heroic old hillsbrad, just kill the new bosslet (forget his name) with the dog pet. He's super easy and the blue version of the mariachi hat is shardable. Zero trash fights to get to him, he's just on the road patrolling around.

You can soft-reset him by exiting instance, switch to normal mode, back to heroic.

Come to think of it, first boss mana tombs is probably best bang for your time. A few trash pulls and you're on him, and a quick run in/out for resets.