While cleaning the temple there is a gnome all the way on the top. You need maake sure its still on the top when you are done cleaning.
I accidently knocked it off and it fell all the way to the bottom. Since you dont have to be precisely next to an object just try to find points that are low enough so you can grab the gnome and put it back up. Try this step by step. If you cant pick it up you have to get closer to the gnome. When all the way to top place it back on the top.
Watch it because the gnome. Unless you happen to pick it up and it can land on its feet. There is no way to rotate back afaik. Layingthe gnome flat on its face or back still triggers the achievement.