*Edit - 3.0.3 updates
As of the current Wrath live build (3.0.3), this quest is two-mannable with a tank and a healer. I just finished it with an 80 Protection Warrior and 80 Holy Paladin. Both characters have standard Northrend 5-man Blue/Green quality quest rewards.
Landgren is in the Cathedral at Onslaught Harbor - it's hard to miss.
Clearing to the Archibishop is reasonable. If you do like I did and just charge in the front door while on your flying mounts, you'll end up with 4-8 normal level 80 mobs - a couple Onslaught Paladins, Harbor Guards, Raven Bishops, and possibly a Gryphon Rider or two.
The main chamber has 2 packs of 5-mob pulls - 1 Raven Bishop, 1 Harbor Paladin, and 3 Harbor Guards. These will respawn at a standard mob respawn rate (I'll estimate it at 7 minutes for now). Their respawn also seems tied to Landgren's respawn, so you'll most likely have to clear the room again if someone kills him shortly before you arrive.
Landgren himself is accompanied by two normal Harbor Guards. Landgren has roughly 300k HP with 80k mana (essentially endless for the purposes of this encounter). He melees for 1200-1500 on a warrior with 20k armor, Mind Blasts for about the same with no resists, has a Shadow Word: Pain that ticks for an unknown amount (thanks Cleanse) and has a Mind Sear that ticks for under 1k for several seconds. His Mind Sear is otherwise like a regular priest's - it has a 10 yard AoE effect around the target. He is unstunnable and uninterruptable. At this time, you can Spell Reflect his Mind Blast and Shadow Word: Pain but not his Mind Sear.
At 50% life, he switches to Shadowform gaining 25% damage to his Shadow spells.
As you may have guessed, Shadow Resist is effective against all of his spells.
The fight itself is straightforward - no Fear, no Mind Control, no special tricks like that. The only issues to contend with are the possible respawn of the interior guards and the AoE of Mind Sear.
To deal with the possible respawn, you can bring some DPS. 1 competent DPSer -should- be enough, 2-3 would be safe. I had Landgren around 30% life with all the DPS coming from the Prot Warrior when the room respawned.
You can also do like I did and pull Landgren to the alcove (You have this word, yes? Alcove?) before his main chamber. Your healer/ranged DPS can stand in one corner to avoid Mind Sear while your tank/melee hold Landgren in the other corner. You should only have to handle a respawning Raven Bishop or two if you stay out of line of sight of the main chamber and the guards towards the exit of the cathedral.
To deal with the Mind Sear, just have your healer/ranged keep their distance from Landgren. Note that if you fight any Paladins at the same time as Landgren, they have a Charge/Intercept which can interrupt casting. As of this time, the Charge/Intercept does not spell-lock like a Kick or Counterspell.
Once Landgren is down, be sure to quickly use Arete's Gate to finish the quest and to grab the follow-up quest. Landgren himself doesn't really have any loot, so it's best to use the gate as soon as you kill him.
All in all, it's an easy fight provided you have a plan to deal with the adds and you use the gate right away.