For the first one, I've picked 'Sniper Duel' from Minecraft (X360 & XOne).
Kill a Skeleton with an arrow from more than 50 meters
Firstly, one block it's equal to 1 meter, so you'll need 50 blocks between you and the skeleton, at least. On Easy, it take up to 3-4 hits to kill an unarmored archer, so bring as many arrows as you can (and save before you start).
Make sure you measure the 50 squares before, so you don't need to do so when Creepers, Zombies, and other enemies are out.
When I earned the achievement, I setup a pen to keep the skeleton from chasing me, as they usually do.
A small pen is fine, 2x2, 2x3, 2x2, and the last side should be open. Use fence posts like I did to trap the skeleton in, leaving it open lets you trap him easier.
When nightfalls, lure a skeleton to the pen and smack him in, quickly finish the fence, and he'll be trapped with no escape. >:)
You'll need to aim a good distance above the skeleton, 3 or 4 crosshairs worth. Keep trying until you kill him, and if you were 50 squares away, you'll get the achievement.