Elder guardians usually only spawn in ocean monuments or in oceans. It can be hard to find and kill one in that way.
You can easily get this achievement in an achievement world.
I've added SeangamerPro's world to my dropbox file. This world (0.16_Achievement_world) has all 0.16 update achievements in there. For this specific achievement, get armor and potion from the Wither chest (back left). Then get the Elder guardian spawn egg from the chest in the back. Spawn one elder guardian, and kill him to unlock the achievement.
World can be downloaded here:https://www.dropbox.com/sh/v6ggssr1ag7uerp/AACU3QfRen68hOMno...
This forum post might help if unsure how to transfer worlds to your PC/device.Achievement worlds