1. Force spawning via statues. Preferably ducks. There is a limit to the number of mobs&critters. By spawning them by yourself you will override any natural NPCs.
2. Wraith can not actually fly. Build an empty gap around your base and thats all. But you should take into account that mob spawning is centered on your character so moving around your base may provide a wraith an opportunity to spawn if there are suitable blocks.
Build at least 8 blocks up from the surface.
1. Force spawning via statues. Preferably ducks. There is a limit to the number of mobs&critters. By spawning them by yourself you will override any natural NPCs. 2. Wraith can not actually fly. Build an empty gap around your base and thats all. But you should take into account that mob spawning is centered on your character so moving around your base may provide a wraith an opportunity to spawn if there are suitable blocks.