Early expansion achievement is easy enough. Just go into a custom game and 14 hatch should do it. Or just go vs very easy AI on a novice map and just save up minerals and do a 6 hatch.:P I can't remember but I think I got that as zerg doing like a 12-14 hatch.
Just get more vikings and more marines. I remember doing that mission just crazily making tons of vikings.:P But going marine medic can basically do almost every map if not all map on campaign even on Brutal.
If you want to sight them then just get orbital commands before the mission and use scans then laser beam them. Piew piew.
Challenges are discussed in this guide
http://sclegacy.com/feature/2-encycl...ngs-of-liberty
The 8 hour achievement is just total mission time. Just run through the campaign. You don't need to do all the missions. Only need like 19 or something missions to complete it. Just rush through and it's pretty easy. I did it on my first run through to get the 8hour campaign completion achievement.