Finding all the mooses. Er … moose? Meese

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.

There are 3 Defiler bones that are scattered across the map:

• One bone is located south of your starting position, and you can see it from your starting position.

• The second bone is located to the northwest of your base, near the Train tracks. The third Defiler bone is located directly south of your starting position, next to the bottom Train tracks. The first two are revealed on your mini map at the start of the mission.

• The third is revealed by the second train. In all cases, they are all revealed by the 5 minute mark, and none of them are guarded, at the beginning of the mission. Get them as soon as they are revealed, before it becomes harder.

Source: TL Wiki - Starcraft II - Campaign - The Great Train Robbery