removing paint from wall.

I removed some nails from my wall, put in some spackle, and repainted. However, I used one two many coats and need to sand it down and redo it. What's the type of sandpaper I should use? Also, is there any other mechanism I can use to make it a little easier. I'm using 150 sandpaper and it doesn't seem strong enough.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.