Need to do these first:
http://classicdb.ch/?quest=7581
http://classicdb.ch/?quest=7582

You get the materials to make the "prison" that holds your Doomguard.

You should be able to solo the quest without too much trouble. The hardest part is being able to get to a good place to find a doomguard captain alone.

They are to the south of where you get the quest. It would be a good idea to get a Voidwalker out now. If you have an epic mount you can just ride though all of the mobs without too much trouble. Be careful though, those guys hurt and can hit for over 1000 damage, and if you get dazed you are pretty much done for. Try to stay as far as way from them so you dont get hit as you pass them. Find a good place where you can just aggro one captain alone. Send your voidwalked in and just start using the item, your Voidwalker will probably die by the end, but by the time he does you should be done using the item.

Also after you finish using the Prison you need to pick up the item that is right next to corpse or else it won't give you credit for the quest.

The hardest part about this quest is getting into the necessary position. It's difficult to explain but I will try. Before doing anything, make sure to take off your gear- you can die ALOT trying this and you don't want to bother with repair bills.

Basically, beginning at the entrance to the Tainted Scar (you shouldsee 3/4 elite arranged in front of you) you are going to move straight forward towards the west which will be taking you in the direction of some hills/mountains. Now you should see a narrow-ish path leading up these hills/mountains with a guard off to its left and another further off on the right- you won't need to worry about the right, just wait until the left has pathed as far as away as possible and then get up the path. Moving up the path you should see a few small hills on your left, move up onto these and start making your way South/Southwest along them. You will know if you are in the right area if there is a group of 2 Doomguards and a Dreadlord all standing together on your RIGHT side. Now, when you have the group on your right, you should be facing some steep slopes (Still being south/southwest). You can get over these but it will take some work- don't give up- just keep trying to run up the slopes as far as possible, turning slowly in either direction until you find the right path upward and over them. You should now have that group I mentioned (the doomguards/dreadlord) behind you. The rest is fairly simple. Dismount, put on WALK, and move in the same direction you have been, bearing to your left. You should make your way alongside a steep hill and shortly see a single, lonely doomguard. He has no friends to help him out. There you go.

I did this by having a healthstone/pot ready, as well as being DM specced and 1. using a VW sacrifice 2. using Fel Domination -while- I had my VW shield still up and summoning a second VW to sacrifice again during the fight. Basically I had a second shield on hand. Now the DG will just stand there or even wander around normally for the first 3-4 seconds of channeling, and then come at you head on. Don't bother trying to draw off Aggro w/ your VW because it won't work. The only way the DG can interrupt you is with a war stomp- and if he does that then I'd recommend just dying and retrying because it'll be very difficult to restart channeling and begin again whe he's already in your face. If you're not DM specced that's fine, you can still sac. one VW and then summon another- you'll just have less time to go on your first VW shield, but they only last for 4-5 hits anyway.

Also note: the crystal only channels for 15/20 seconds, but it will be 2-3 seconds AFTER it has finished channeling that the DG will turn into the crystal you need and can pick up. So if your crystal finishes channeling but the DG kills you in those last few seconds, don't worry and don't send in a GM ticket because nothing is wrong- it's just lame like that. Also remember to PICK UP THE CRYSTAL- it is fairly small and easily missed if you're too eager to race back to the quest giver.

I hope this helps anyone out who had as much trouble as I did. =]

Also to note- if you don't want to solo this quest and you don't want to find this singled-out DG then I would recommend a raid of 10 or more. Yall can say what you want about those demon elites, but a 5-man group won't likely cut it unless you are very geared out. They do blast waves for nearly 1k dmg, hit very hard and have a large amount of health. Basically the juiced up equivalent of Onyxia Warders if you've ever fought those. I tried getting a large group of 10 for them and it was slow-going. Anyway, good luck and have fun summoning the DG. It's not really practical but fun to summon randomly.