It’s a Matter of Choice: Campaign Difficulty
The StarCraft II: Wings of Liberty single-player campaign is more than just a continuation of a legacy. It also offers new and veteran players alike the opportunity to challenge themselves, while learning the mechanics at their own pace.
When starting a new campaign, players choose a starting difficulty:
Casual: You have little or no experience playing strategy games.
Normal: You have some experience playing strategy games.
Hard: Only choose Hard if you are a StarCraft veteran.
Brutal: Only choose Brutal if you’re a StarCraft expert.
Don't worry, you're never locked into a difficulty setting. Prior to taking on a new mission, players are given the opportunity to adjust the difficulty. Whether you want to tone down a difficult mission or crank things up to see what you’re made of, it's up to you.
Players who feel that their skills have improved can access missions they’ve completed on the command console on the bridge of the Hyperion and replay them at a different difficulty setting. In many cases, Achievements can be earned for playing missions in either Normal or Hard difficulty.
The campaign can be experienced in many different ways. For some, playing on Casual difficulty is about experiencing the story the first time through and then going back to replay on a more difficult setting. For others, nothing less than Brutal difficulty will do.
There’s no one way or best way -- just your way. How have you decided to take on the single-player campaign?
For those looking to take on Brutal difficulty, Alleji's guide has a few helpful tips for taking on some of the more challenging missions.

Then I got to the final level. This was almost cake when I took the flyer route: it was incredibly fun and I finished the achievement with 23 mutalisks and 13 brood lords under my command. Cheap, easy, but fun.
This time I tried the nydus version, and it is MUCH harder. It spawns SIGNIFICANTLY more enemy units, and they can't be countered as easily as aircraft. On top of that, you have to send units out of your base, even out of the range of the nova to kill the worms. Now perhaps the psi disruptor would be better for this mission than the hive emulator, but still the two missions shouldn't have such a gap in difficulty between them.
Of course, once I adjusted to the new difficulty it was fun as well...in its own way. Reminds me a bit of the Warcraft III 'World Tree' final mission, the best level in the history of RTS games in my opinion.
Lack of air attackers means a sufficient number of siege tanks can solve all of the mission's problems...
I actually am happy about this because it has given me a chance to enjoy the story and get achievements with little frustration. If I want a challenge when I finish on hard and normal I'll try brutal.
And no, don't take this as "bragging". I'm horrible with terran and I know it. I'm just curious if anyone else has noticed that M&M spam makes winning missions on hard simple even for novices.
1. Normal, where I unlocked the first two achievements.
2. Hard, where I attempted to unlock the achievement... but far too much is happening to make that easy to do. Maybe if the zerg didn't cheat. To proove cheating, I then did
3. Casual. Where I completely wiped out every last trace of the zerg, put bases where their three bases are, and they still appeared for the last 1:30 of the mission. My marine force taking them out as they showed up in their nydus worms.
So, now I know the layout, sort of. But I'm getting a sense that it goes more like:
Walk over everything --> Moderate --> Micromanage fest as we start to throw everything we have at you --> You probably are insane, right?
You'd figure it would actually scale Casual --> Normal --> Hard --> Brutal like the names imply. I'd love to have an inbetween Normal and Hard, personally. I can't even fathom what brutal would be like.
But hey, at least there are scales. I recall missions that I simply hated in Warcraft 2 and beyond because the odds were way over stacked. So I guess it's nice that we can still set it to easy-mode and finish it up.
But then to tantalize us with achievements? That's like saying: you can play how you want, but look at this reward we've put out of reach.