The fight starts out with an OffTank fighting Midnight. Let them get argo and then have at him. When Midnight gets to about 95% health the Huntsman will come out and that is where your Main Tank will pick up Attumen. In all the raids I have been in all the DPS stays on Midnight.
Once one of them is down to about 30% health all fighting will stop and they will join together. During this time your Tank will position themselves and you will get up "right on the horses ass". You want to stand as close as you can get and still be range. One step forward you will melee. This is very important and take the time while the tank is getting argo to get just in range distance. You do this because he will charge at anyone who is farther away. You will take damage, be stunned, and your raid party will probably move a little during this time but he will come back to his same position. Keep DPSing hard and FD when you get high on the Omen charts. This fight needs to go fast because if you have entry level Kara Healers and Tanks they will be running out of mana quick so they are depending on you to DPS your heart out.
Attumen have loot that makes Hunters cheer. You will probably be wearing a good part of this gear for a while. It is an easy fight and you get one Badge of Justice from him.