This achievement (Meet New People. Then Kill Them.) is for winning a multiplayer match. In ranked matches, you get 10,000 "points" that you spend on a team of humans, aliens, or a mix of both. Tougher aliens cost more, and so do soldiers with more experience and skills and better weapons.
Whether you win a match or not depends a lot on your opponent and what they choose. I seemed to do best with the following team:
- Sniper with archangel armor, plasma sniper rifle, and one of the skill loadouts with Squad Sight and Damn Good Ground.
- Heavy Floater
- Thin Man
The Thin Man and Sectoid were purely for scouting, and I kept them as covered as possible. While my opponent was distracted with them, I flanked them with the Heavy Floater, and once they came out of cover to attack him, my sniper (who was flying high all the way across the map) made short work of their units.
What style you choose is up to you. There is no "winning" strategy, since any strategy you choose can be countered easily with the right setup. Just have fun, multiplayer is actually pretty amazing in this game. You'll win a match eventually.
Edit: Apparently you can get this in a private match as well, thanks Shinwasha. In private matches, strategies will differ, but I imagine most private matches will be to boost this achievement, so strategy probably won't matter anyway.