That would be the ultimate achievement for me, but my micro skills aren't up to par :P
Did you rely a lot on the mercenaries on that level? Assuming I believe you that is ;-) I have managed to beat the level on hard, once, and with a severely damaged nightelf base

You have to start with the mercenaries at the beginning. Immediately. Then get the Goblin Workshop, buy a bunch of mines and place them in appropraite places. Then move treant towers to the entrances, and several wisps that would stay in the back and constantly repair. Then it is all about micro and using Tyrande's Starfall and Malfurion's ultimate at appropriate times when you need more damage or more healing.

Focus all your defense on the Human base. Move all buildings except the Tree of Coins down whenever you don't need to produce units, so that their pathing is shortened.

EDIT: It takes practice. But then, I have completed the game 10+ times, so..

Interesting. I had given up on ancient protectors due to the stupefying amount of meat wagons that accompany later attack waves on hard. But I'll try that next time, with a +-5 wisps to keep them alive. They do inflict pretty high piercing damage from what I recall. I probably just need more practice. The landmines for example: I never could find time to place them in between all the preperations I had to do between waves, and I can imagine that when placed cleverly they can have a significant impact.

There's one mission though that was harder for me to beat on Hard than Hyjal : that blasted one where you had to reach the Horn of Cenarius before the timer runs out.

You don't need many protectors at the beginning. Only a few to soak some damage and attack undead that come into melee range of the ranged support units.

Then when you're at your population limit, you try to build as many protectors as possible.

Pop limit? So you went into high upkeep on that mission?

The end mission in Heart of the Swarm was super difficult on Brutal, especially if you're going after the achievement at the same time. I had to go down to Hard to get the mission-specific achievement and then complete it again on Brutal for the campaign achievement.
Yep, but then, it is important to not max out immediately, so that you could get resources faster to do the upgrades and build as many protectors as possible.

What concerns me is that after how ever many years of World of Warcraft, they won't know how to handle it again in an RTS format if they do make a new one. What scares me worse is that they will get desperate for the money and turn to Diablo and Starcraft to form new MMO games instead.

What do you mean?

The team that is working on WoW now is not the same team that is developing Starcraft 2. If they make a Warcraft RTS, it would be the Starcraft team that would do that RTS, with the Heroes of the Storm game director Dustin Browder leading the charge.

They are not going to give an MMORPG dev to create a competitive RTS when they already have a massively experienced RTS team in their house.

Yep. Big YAWN to blizzard games, Warcraft 1 and 2 were their only good games, ever, IMO, and they were just that, good, nothing special.

I don't know why anyone would be excited about a possible Warcraft 4 when almost certainly the campaign would be written by hacks like Metzen who can't write exciting fiction to save their life

After the bullshit of making Diablo 3 and Starcraft 2 always-online DRM games, fuck 'em. I don't even consider them as existing anymore.

Oh, I wouldn't say that. I loved Diablo 2, Starcraft and Starcraft 2. But if I were to play a new Warcraft game, I'd prefer one in the style of Warcraft 2 rather than 3. Sure, make it full 3D and introduce modern controls, but lay off the hero-centric gameplay, lose the horrible level scripting and return to the humorous approach.

Edit:
Oh, and The Lost Vikings was awesome too ;-)

This is great news unless Warcraft 4 ends up being a MOBA or something similar.

Spellforce games are definately better at mixing heroes and RPG elements with RTS than Warcraft 3 was. Still, I think there was a lot of good stuff in W3. Warcraft 4 should be a mix of both W2 and W3 with some new ideas.