Just do what Fur said. Spam your Greater heal on your tank, stun one and open fire. First time i stepped foot in slave pens I got my ass handed to me also. The difference between normal dungeons and heroics is so large that you can't just jump into it with the same kind of reaction times that you used on normal modes. You've gotta spam everything. I have an all in one paladin tanking macro that i mash that's tied to my DPS macros.

/castrandom Holy Shield,Consecration (Rank 1),Cleanse

If you are losing agro adjust the Consecration rank up, but you'll burn too much mana if you go max rank probably. Just tinker with it. Installing a threat meter can help you see how much agro your tank is pulling compared to your DPS and then you can fine tune the spell ranks.