The withered is a random world daily that can cycle between 2-6 days. Frosty and I have been sitting at every 2-3 days, the daily will show up. Here are some tips, and videos for you guys. Most of this has already been said in guild chat.
Before doing the withered the next time it cycles through, get all of your mana capped. The current cap for most guildies is 9/10 on the achievement. http://www.wowhead.com/achievement=11133/why-cant-i-hold-all-this-mana Use TomTom, which has been updated, to get coordinates of the various items.
(Side note: Make sure you finish your leylines and your portals, as well. These are very helpful and will give you reputation.)
The best place to farm your mana is in the vineyards in Suramar City. It takes about 3 minutes to cap at 2k mana.
1) This is not a timed scenario. Take your time. Seriously, take your time. The longer you take, the better chance of you to remain calm and not lose as many withered.
2) Focus on chests and kill smaller adds. Once you loot a big chest (say requires 10 withered), it is gone for good. So, if you get the chest that requires the 10 withered to open the door (near the beginning), you do not need to go back inside of there to loot it again on your next run.
3) Use Touch of the Void. It is currently glitched. You will find this in your heirlooms page. Use touch of the void on those big void explodey guys.
4) Big void explodey guys need to be tanked away from your withered, and facing away from them. This way the ball the add does, will not hit your withered, and the explosion will not hit all of them. Use touch of the void at around 75%, silence as much as possible, and stun at 25-30%.
5) Only hit the withered extra action button unless you find yourself in a pickle, IE: the elite/rare add suddenly pops up and kills half your withered. Most of your withered will auto-attack and auto-follow you, without hitting the button. When you hit the "go crazy" button, the withered actually go crazy! They will pull everything, even stuff below you, because they are insane. (They do 75% more dmg when in their crazy mode)
6) To open the berserker door, you need to loot the berserker helmet from one of the chests inside. Upon starting the scenario the next time, you will gain a berserker in your withered army...and from now on.
7) Unless you need the key, try to avoid the elites as much as possible. They rape the withered immediately (like the stealthy one). The stealthy guy can be bribed and used to attack elites later on.
8) If you are a healer inside, you can heal the withered...but if you are not killing anything, the withered will not regain their health and will continue to tick to death.
Onto the Nightfallen rep:
Here is a great guide for it. It will list all of the quests that you can do, and I strongly suggest to do them all. You will need 8000/12000 rep to finish the final two arcs of the storyline. You need 8000/12000 rep to go into the final 2 mythic dungeons. (Update: It has now been changed from 5k to 8k for the new rep chains)
That is the achievement to track your progress on your story arcs. In honored, you should have completed all by Statecraft, A Change of Seasons, and A Growing Crisis (currently glitched, and you will not receive credit for it until 5000/12000 rep).
Here is the guide for the Nightfallen rep:
(Also, this first comment on this page is very useful to tell you exactly which quests are in what storyline arc: http://www.wowhead.com/achievement=11124/good-suramaritan#comments)