get your tank to 535 min defense
there are pleanty of ice crown daily, ebon blade has nice easy daily set in particular. ( 6 total but one of them is an awful collection quest that I don't do )
Buy your tank whatever it takes to get to 535 defense. ( at least that's the magic number for a pally ) I'm sure others can give you ideas of what a DK needs for defense.
for heroics I have one of my shaman as resto doing zero dps, heals only on a cast sequence that starts with earth shield, riptide, lesser healing wave.... ( as many lhw,ch as it takes so that when I get back to riptide it casts right away )
so for button 2 I have 3 shaman doing dps and the resto just standing there and
for button 3 I have the same dps but the resto does his healing castsequence on the tank only.
and the numpad-5 I have all shaman do a chain heal on themselves.
most times when the group needs a heal I can just tap the numpad-5 and only the resto is ready to cast so only the resto heals the group although it is my panic button and if more cast chain heal it's usually ok.
The easiest boss to take down is the right hand side boss in gundrak, the one that turns into a mamoth. He does an AoE stomp that hits the whole but he never double stomps so you have plenty of time to heal your tank and your team before he stomps again.
the easiest last boss is in Zul'Drak because 1/2 of the fight is in transformed mode.
good luck.