In a word: don't. At least not for individual achievements.
For mission-specific achievements, create a redirect to the achievement section of the mission. Put the achievement into its own subsection if you want to add tips. Remember to keep your writing tight.
For other achievements, group achievements together in an article for each "area". See what I have done for "StarCraft II: Wings of Liberty achievements".
Achievement icons for Wings of Liberty should go into "Category:Wings of Liberty achievement images". See examples in the category for naming convention. General good image uploading practices continue to apply. - - Meco (talk, contribs) 15:31, August 2, 2010 (UTC)