Starcraft 2 WoL achievements in HotS

I believe that it does have to be a blizzard sponsored map. Easiest way to get it in this case is to play on Lava Flow as terran. As soon as the game starts, load your scvs into your command center and fly to one of the islands. Get an engineering bay pretty soon, and start to pump out turrents along the edges, and go strait to battle cruisers. Soon there will only be one AI left, and you should have close to 20 cruisers, so you should be able to steamroll.

At first, the credits available to you in the single player campaign is limited to that from missions alone. However, once you've fully maxed out either the zerg or protoss research trees, you gain the ability to trade any future research points into 10k worth of credits.

There have been mentions of a glitch with the way the game handles this: if you replay a mission you've already beaten and acquire protoss / zerg research points within that mission, it is possible to get the Moebius foundation to "purchase research samples" from you, even though you failed to increase your net points (you already got them once). In this way, you can accrue as many credits as you need, and can buy all the armory upgrades in a single playthrough.

Update: This method has since been patched.

As of now, the best way to earn the armory achievements is to play the campaign multiple times, selecting different upgrades on each playthrough, as achievements are shared across all campaigns.