OK, let me do this correctly and not just posting the obvious. In all seriousness.
OK, the games achievements are this. I will post how hard they are, and how long it SHOULD take to accomplish this. The multiplayer wont take you long at all, cause by beating all the campaigns you get experience, my friend never touched the multiplayer and got 6000XP for beating the campaigns. Just do the rest yourself, and by the time you hit 18060XP, you should have the rest, if your good, the only one that would give you trouble in my thoughts are the The Uninfected achievement. There is a good guide on that achievement, just follow it and you should have no trouble with multiplayer. The ten kills can also be gotten in Infestation. Getting 6 friends is probably a hard one also, cause when I did it, we got matches like 7 on 2 and crap and they always left before the game started, so try to get this one as soon as possible.
Now, the hardest achievements are in singleplayer, beating all campaigns on NIGHTMARE difficultly is a real fricken nightmare. The marines should be your biggest problem here, but if you can do it, more power to you. (UPDATE) A guide was just put up for the Marines campaign, although it isn't a video guide, it has good tips. So maybe now that a good guide is up for it, maybe more people can get it! The other campaigns are easy on nightmare, it is basically hard with no checkpoints, just remember that. If I were you, go through them on hard first, get used to the controls and the levels and what your supposed to do. Then once you beat it, change it to nightmare and go at it. Aliens and Predator has guides. Video guides that is.
Now, if your not a fan of collecting stuff, then have fun with the next batch of achievements. All campaigns has stuff to collect. Predator has the belts, which is easy cause if you use your vision, you can see them. Marines has the video logs, which has a guide now, and Aliens has the gooey canisters, which also has a guide. The killing all marines and harvesting all humans can be difficult if your not a good Alien, trust me on this. I had the roughest time getting use to the controls. Oh, I forgot, everything you collect goes onto the next play through, which is great!
Ahhh, Survivor, this is fun and doesnt take long. Have you four friends and the amount of enemies doubles, go to C BLOCK on it and just grab the smartgun and fire away, the health will come naturally if you survive long enough.
Weapon kills arent much worry. Getting two kills with one battle disc can be a breeze in the campaign, just find a huddle of enemies, and swing it. I got this with the androids at the end of the game. The two kills with the shotgun I got in survivor, use LT for this. Smartgun don't even need to worry about. Combi Stick is fun to use, so you shouldn't worry, and your alone with your pistol for the first part of the Marines campaign, so you'll have the kills with it in no time. Sniping enemies can be a bit of a problem, I waited until Acid Aliens showed up, cause they stop and spit, so when they stop...POP their head!
There you go, the best I can describe on how to get this achievement.
(UPDATE. IGNORE THE XP GLITCH. DEVELOPERS HAVE PATCHED THIS.)