Bring all the materials once you present him the Libram.
This way you will save yourself a lot of time (just stating the obvious here I know). :)

This quest at first seems pretty daunting, but it's actually quite simple and just a little time consuming once you know how to acquire all of the mats. The effort is well worth it, having a +100 hp enchant to your legs and head piece will make a big difference!

First, night dragon's breath can be acquired in Felwood. Depending on whether your Alliance or Horde, you'll have to talk to the appropiate NPC to begin the cleansing of Felwood, which will involve you kill the Warpwood Shredder's in the northern part of the zone to collect shards. The Horde NPC that begins this quest is right off the road, a little further north than the druid tents and to the left. Once you've completed this first stage, you'll be given a Cenarion Beacon (hang on to this, you need it to collect corrupted soul shards!).

Now you must collect six corrupted soul shards and in exchange, you'll receive two salves (And you need four to collect night dragon's breath). Corrupted soul shards have a chance to drop off any mob in the zone. Once you have four salves, you can cleanse the night dragon's breath plants. They are located at the following coords:

1st Location: 35,60

2nd Location: 50,30

3rd Location: 40,78

4th Location: 42,13

Once you've cleansed the plant, you can collect the dragon's breath (usually yields 4-5 per use). Once you've collected from a plant, it takes about a half an hour before you can collect again. The nice thing is, if you hang around and wait, you can just keep collecting, you don't need any more salve! So I'll usually just hang around and keep collecting from the same plant.

The rest of the quest is easy. Hop on over to the Blasted Lands (Horde have a flight path to Stonard in Swamp of Sorrows, Alliance has Nethergard Keep right in the zone I believe). A blood elf in the ruins near the entrance gives the quest for the lung juice.

The scorpids are right near the entrance with the hyena's and vultures and the drop rate is fairly high. A little further south near defiler's rise you can find the basilisks, who also have a fairly high drop rate, along with hellboars, whose drop rates are quite a bit lower (This is what you'll be grinding on for the longest time). Remember when you're grinding that the hellboars, basilisks, and felhoudnds all share the same spawn point, so kill everything, it'll keep the hellboars respawning much quicker! There's also another variety of boar further east that will drop the blasted boar lungs as well. Once you've gathered everything, hope back over to Mathredis and you're done!

Is this quest repeatable? (I want to know if I can have the enchant on my main and my twink)

you can't just go to him and take the quest... you'll need the libram to get it!

That picture is amazingly demonstrative! Thank you, chrisfrost from coilfang!

Anyways.. I did this quest with my 64 enhancement shaman, and I dont know if I should slap it on my alt or keep it for my Headdress of Inner Rage which is sort of lacking HP.

I looted the Libram while grinding, and I had a couple of Black Diamonds in the mail which I couldn't get sold via AH, so I decided I'd get this, its quite worth the effort since the only head-enchants youd be getting at lvl 55+ are ZG-enchants.

lol the screenshot is perfect! I did see where that was instantly, good pic there ;D

Epic screenshot.