Allright so I found out another useful thing for DH's (if you haven't seen the smokescreen post just look further down in the D3 exploits section), this one a friend told me. On bosses, or possibly large/large hitbox creatures, Elemental Arrow with Nether Tentacle rune hits twice, meaning you effectively do 310%(2x155%) weapon dmg for only 10 hatred! Together with Marked For Death with Mortal Enemy Rune (3 hatred per hit), you can get insane hatred recovery too, allowing you to spam Elemental Arrow for a longer period of time. I've only tried this out on Belial but so far this has worked.
Edit: This also seems to work on a bit smaller enemies if they stand still while the tentacle orb passes through, maybe this has to be the case for bosses too or they just have a hitbox so big it doesn't matter. Regardless, if you have a tank or some slow/snare/stun it should still give you some nice dmg. You could also use smokescreen (so enemies stand still for a few sec) to get those double hits.