I got the first milestone more or less honestly, and then farmed the second one. As good as doing things "honestly" felt, going on an empty server with 5 friends, figuring out how the achievements work, taking turns in getting them, and constantly communicating was the most fun I've had with a game in ages. The third is going to take a while because of the 1M healing points, though...
Congratulations to Valve for inventing an interesting new gameplay mode like Achievement Farming! I can't wait for the next mission pack. =P