I was doing some achievement-hunting on the weekend and decided to knock out The Keymaster, since I needed only two keys to finish it - the Shattered Halls key and the Key of Time. I was 2/3rds through friendly with the Keepers of Time already, having soloed Escape from Durnholde a few times. I quite enjoy Durnholde, it's a fun lore one, and it's funny to see an undergeared teenage Thrall, so I enjoy doing it and just spectating - it's like a vacation in WoW history! Him roaring at the horse to scare it away always make me smile.
I had never done the Black Morass, where you have to defend Medivh as he opens the Dark Portal for Gul'dan. This is partially because it just didn't sound as interesting - yay, let's go defend some unlikeable, pompous twit in a swamp! A human, at that. Feh! But, my friend told me finishing the KoT questline that ends with Black Morass gives you enough rep to get to Honored, which is needed for the key, so in I went.
In short, Black Morass is a lot like Violet Hold. But in other, crucial ways, it's different. I found this out the hard way...
Just like Violet Hold, Medivh has a percentile health, starting at 100%. It's like the shield strength at Violet Hold. You know, that 100% that never goes down? Apparently if the spawning dragonkin reach the door they start breaking the shield. I say apparently because...I've never, EVER seen it happen.
So anyways, Medivh has a shield. At various points around the swamp yellow flashing temporal gates will open and some Infinite Dragonflight guys will pop out and make a beeline for Medivh, trying to break his shield/kill him.
This is where it's a little different from Violet Hold. I started picking off the guys as they rushed in, and it was easy. Like faceroll easy. Didn't even need to use my pet, just blew them all up as they approached.
However, the waves started coming faster and faster, and I eventually become overrun by an endless swarm of whelps and dragonkin. I feigned and reset the instance after Medivh bought it, and tried again.
Turns out that instead of just killing the waves (like in VH) you have to actually go find the elite who is standing at the time gates, and kill it. Then no more mobs will come from that gate.
Also, you can't just send your pet at the elite to aggro him and then drag him back to you - they're chained to their gates and will reset if they get a certain distance away.
Once I realized this, it was a cakewalk. Locate gate, kill elite (I think they're called Rift Keepers), kill any remaining mobs before they can attack Medivh, head for next gate. After 5 or so gates, a boss will spawn, just like Violet Hold. None of the bosses did anything of note, though like all Infinite Dragonflight guys, they all had some chilling/cool prophetic lines. Simple tank/spank kills.
Any mobs headed for Medivh will forget about him completely if you damage them even slightly. Before my first attempt spiraled out of control, I was keeping 12+ mobs off him through my Bear's Thunderclap, Multishots, single-tick Volleys and Explosive Traps.
The other good news is you don't need great gear to solo this place. I used my regular PVE spec and regular PVE gear, and neither myself nor my pet ever really took any significant damage, even when I myself was tanking half a dozen mobs during attempt #1. On my second try, if I was killing a Rift Keeper and a mob popped out and started headed for Medivh, I would just Serpent Sting him and let him claw at me for a bit while I finished off the Keeper, and didn't take any noticeable damage.
EDIT: I was wrong, I was in my soloing spec after all.
Once it's all done, head back to the quest giver at the instance entrance and he'll give you a quest to talk to someone outside (in Caverns of Time, not outside-outside). Go do that, and you'll receive an insane +8000(!) reputation with the Keepers of Time. Chances are that will push you to Honored, and you can go get your key.