Starcraft – How exactly can you get the “Liberty Completionist: Hard” achievement

I managed to do it with just Marines, Medics, Marauders and Siege Tanks. I put up two bunkers and four tanks at each ramp for defense, then built up my forces for the assault. I attacked the base directly to the north. The initial tactic is to place tanks in siege mode a little bit back from the protoss ramp so they are out of range of photon cannons. Then run your MMM ball halfway up the ramp so the tanks can start shooting down the cannons. Meanwhile keep the drill hotkeyed so that it can assist in taking down archons, collossi, void rays, and anything that's doing damage. Don't let your MMM ball run out of reach of your tanks.

When you've beaten back their initial defensive line you've got a foothold. Move your tanks up and keep on pressing in. Focus on taking down the pylons around unit producing buildings and keep your drill handy to zap the occasional pylon or counter-attacking heavy unit.

The real key to this strategy is you need to keep reinforcing. Keep your barracks and factory hotkeyed and move the rally point to deliver troops near the front as you move from left to right across the map. Before you've taken out the entire northern base you should have the achievement. Good luck!

Yes. Any achievement, any achievement, that has a difficulty requirement as part of the achievement can also be completed on a higher difficulty. This means that (if you're good enough) you can get all of the Wings of Liberty Mission achievements on a single playthrough.

E.g., for the Red Lobster achievement, you can taunt the Brutalisk into the lava on Normal, Hard, or Brutal -- you don't "have" to be on normal (despite the listing to the contrary)