How to Obtain All Achievements
I'm going to list these achievements in order of what percentage of players have earned them, starting with the easiest and going down to the most challenging! Potential spoiler warning!
Down but not Out
This is the easiest, and first achievement you'll get. After one of your people gets killed, just wait for their 'Recovering' bar to fill and you'll get it.
Build an archer hut and chase down an antelope - the hut is a little costly though.
After you build that archer hut and kill an antelope - chase a skeleton. The best strategy is to have one person chase, then another shoot the monster, swapping roles every time the skeleton starts chasing the other.
The alien is always located far left from where you load in, past the pyramid. Talk to him, and you'll get the achievement.
Survive 7 days. I will link a guide in the next section with tips on how to do this, but you'll definitely get this achievement just by playing this game.
Thank You, Alien
You get this achievement for helping the alien escape after finishing building the pyramid. Again, the other guide linked below can help with this - or ask in the comments!
Build one of each building. It is much easier to get this achievement AFTER finishing the story, as you no longer have a countdown timer - I verified this to work.
Get 30 villagers. Like the previous achievement, is much easier to get AFTER finishing the main goal. Have a pantry and enough farms to constantly be on full food, and then spam building houses.
This achievement is surprisingly easy - if you don't build an archer's hut, you can also get the 'Pacifist Run' achievement. Focus on surrounding your village with walls and contain any farms you can within them, so you can focus on farming during the night. I recommend doing this on a spawn where water is close to your village.
One of the more challenging achievements. To complete the game in 20 days, you have to be efficient with your buildings.
There are going to be buildings you do NOT need to construct - like the school, pantry, watch tower, etc.
As soon as you load in, send a villager to the bottom left, past the pyramid, to find the alien. The alien is a really important tool as it grants you an extra villager immediately, so you don't have to wait for the child to grow.
You should really be striving to end the first night with a log shed and a hay barn. Try to avoid having a dedicated builder, as making your villagers harvest a resource and then use it to build, then return to harvesting, is more efficient, especially when you only have 3 workers.
Continue micromanaging your workers while you progress through the buildings - unlocking the sawmill, windmill, etc. If people want me to outline strategies for managing your villagers efficiently, I can add that later, just comment it! Around Day 3, when the child grows up, I usually send one of my villagers out to scout for water and for stone.
As soon as you get the mine camp, starting hitting stones immediately. At this point, I have about 5 villagers and the alien, all working - 3 working the farms, and 3 working on farming the stone and building it, and cutting it at night when it's more difficult to go out. Now just micromanage until you finish. You can end the game with only 5 villagers, and the alien.
You can get this, and the 'Vegan Run' achievement at the same time. Focus on surrounding your village with walls and contain any farms you can within them, so you don't have to fight skeletons. I recommend doing this on a spawn where water is close to your village, so you don't have a large area to wall.
The most challenging achievement. You need to be able to micromanage your villagers efficiently, so this might take a few tries and some experience - harvest as many berries as you can at the start - I send two people out to do this, then prioritise building your archer's hut before everything else. Always have at least one person hunting, as carrying the food is a long task.
You also don't really need the farm for wheat, as the amount you need for all the buildings to finish the game doesn't warrant a farm.