Disclaimer: I've only tried this a couple times on heroic and only seen the last boss once. If I wipe after the first boss, generally on the second boss, I leave. Any instance that takes a complete reset on a wipe is bad design in my opinion.

I find the water elemental easier on heroic than I did on regular but I'm pretty well geared now. Anyhow, I run my whole crew up the stairs towards the entrance and park my shammies just to the right and back them up to the little ledge so they don't get thrown and then tank the elemental in the doorway. My macro on my tank includes /startattack which will pick up the closest target while spamming the key. Some of the little elementals I get, some I don't, but it works pretty well in the end.

The voidwalker boss I've only gotten once and just tried to dps race him. It was going well but he got the best of me and I haven't seen him since. I think at the right gear level this is possible though.

I agree the core hound boss is pretty easy.

The ethereal boss also gets me. I do pretty well on her till she or the ethereals (not sure which) summons to the middle of the room then I'm done.