Simple with the new update added for seeds, very easy.
Obtainable on Peaceful Difficulty.
Go to a new world, and go to the seed section and click the arrow. Choose the "Spooky Mansion" seed and choose Peaceful Difficulty if you wish.
Once you spawn up the hill is a Woodland Mansion where Evokers spawn. (They will run fast when hit or sensing danger)
Simply kill one for the achievement to pop and once killed they will drop a Totem of Undying which you can use for the other achievement.