I literally find the design a mess, it can be improved by adding tabs in header and sorting the achievements by categories (General, Worlds, Dungeons, PVP...).
One first tab with achievement points and and status completed of each section (adding progress bars will make it look cool), if there is some blank space can fill with latest achievements acquired for example.
On rest tabs listbox are fine, but I would completely remove the right part, make achievement list more attractive and use tooltips for rewards, reward info or achievement information. Almost forgot, a combobox with sorting options would be great.
If you are planning on make it big, order and sorting options should define the window design.