The Scenic Route
You So Crazy!
Monster Mash

These achievements are Feats of Strength, which means they are not visible to the average person and don't give any tangible rewards, like portraits or points. The first two both take place in the Covert mission chain (Tosh missions) and are on The Devil's Playground and Welcome to the Jungle, respectively. Each of these two achievements requires you to completely annihilate whichever enemy base is present in the mission: Zerg for Playground, Protoss for Jungle. Destroying the entire Zerg base is fairly easy, even on Brutal, as long as you micromanage groups of Reapers extremely well. Destroying the entire Protoss base is tougher, but definitely doable on the minimum difficulty for the achievements (Normal). Note that you do not have to complete the mission to get the achievement, so if you just wanna spend all your minerals in Playground building up a massive force, you can. Just restart and finish for real when you are done.

Monster Mash is a little more difficult. It involves using the A.R.E.S, a robot in the secret mission Piercing The Shroud, to kill the Brutalisk, a nasty hunk of murder that can be released to get more research points during the mission. The A.R.E.S. is very slow and sort of fragile, which means you will want to save it until the big fight, rather than activating it right away. You can even run away from the Brutalisk to activate it, if you want! The only catch is that the A.R.E.S. must score the killing blow against the Brutalisk. Once you do it, you get the achievement.

Insane FFA

Win a FFA game against 7 Insane AI players

Think you have what it takes to take on seven Insane (hardest) AI opponents in a massive free-for-all? Don't worry, it's actually a lot easier than you would think.there's a few keys to doing this achievement: you have to play on the map Extinction, you must be playing Terran, and you must know what you are doing. Basically, you have to pick up and land at the closest island (top or bottom of map), tech up to Battlecruisers as quickly as possible, and then mass them to annihilate the last computer standing. It's easiest if you make all AI players Protoss, as the Protoss ground bruisers are generally ground-focused in their combat. Once you have a fleet of maybe 7 Battlecruisers, you can sweep through and destroy the last remaining AI player and win the match. Getting this achievement will also get you the achievement for all previous difficulty levels, which is definitely nice.

Outmatched Crusher

This achievement seems ridiculous at first glance. Defeating 4 Very Hard AI? 2 Insane AI? You've gotta be kidding! Thankfully, there's a simple method that allows you to get this achievement in a whopping three games. For whichever match, choose the map Megaton, arrange the teams as necessary, and set both your race and the enemy race to Protoss. Remember the Proxy strategy mentioned in the Protoss guide? The AI doesn't know anything about it, as long as you start at the entrance ramp to a base and not immediately next to it. Once you have up a Pylon and Forge, simply push out fast and hard with Photon Cannons. The AI will give up when it has no resources to make a worker and no more workers. Keep in mind you have to do this fast. 10-minute wins are the only real way to beat the harder AI, as their micro and macro is flawless. After you beat 2 Insane, 3 Very Hard, and 4 Very Hard, you should have every outmatched achievement and the Outmatched Crusher meta-achievement.

28 Minutes Later

Complete "Outbreak" on normal before the fifth day

A stunningly easy achievement that is receiving considerable amount of press, this achievement is one of the simplest to get of all the campaign achievements, although you might not get it at first glance. First, do The Devil's Playground so that you can get Reapers, which gain a bonus against both buildings and Light enemies (both of which are common on this map). Throughout the first day and night, produce as many Reapers as possible. Near the end of the first night you should get Hellions, which are completely unneeded and a waste of minerals unless you have no Vespene. Once the first "real" day breaks, split your reaper army into two squads and begin clearing the map. Assuming you are continually moving and attacking, you should be able to clear over half the map on the 6 minutes the first day allocates to you. You can either press on during the night by combining your squad of Reapers into a single force, or smash through after daybreak occurs once again. With their mines and anti-infantyr pisolts, you will likely take no damage on the clean-up effort and be able to finish the level before the third night, much less the fifth. We had 4 minutes left when we finished the level for this achievement!

Hurry Up, It's Raid Night!

Complete the campaign with 8 hours total played mission time on normal difficulty

This achievement, at first glance, seems positively terrifying. After all, it asks you to speed-run the game, which you are probably playing to enjoy at your own pace! However, upon further inspection, there are a few things worth noting. First, it can be completed on Normal difficulty, which is not exactly difficult in the first place. Second, the game gives you 8 hours to finish 26 missions. That gives you almost 20 minutes per mission, which is easily doable on normal. Third, and most importantly, you do not have to do all the missions at once, or even get perfect times on your first try. "Total played time" means the gather the best score of every mission and collate them. If it's 8 hours or less, congratulations, you get the achievement. In other words, just go back and replay levels to get better times as necessary. Some, such as Outbreak or the first mission, are so ridiculously easy that you can finish them in no time at all!

City Builder

These achievements are lumped together in the Economy tab under Combat, and while they seem the most daunting of the Combat achievements, they are actually the easiest. The only requirement for these achievements is to create a build order that gets you what you want as fast as possible. For example, instead of building a Supply Depot then a Barracks, build two Barracks then a Depot during their construction, and train 5 marines on each Barracks once you are set up. To get an early Lair, morph an Extractor as soon as possible, then a Spawning Pool while you gather Vespene and minerals, then the Lair as soon as the Pool finishes. It's all about timing. The best part is that if human opponents won't let you go for it, just play a 2vAI Cooperative. It counts as a League and Melee game for the purposes of the Combat achievements!