4. Always evaluate the weight and fun of an achievement by the most efficient method by which it is earned. Again, players do what's efficient, not what's fun. [...] If there's an achievement for killing 10 enemies without being hit, the player is just going to go to the easiest room on the easiest difficulty setting, find the easiest enemies and get to it. [...] If there's a room full of enemies on level 10, and it's a super awesome expression of skill to kill 10 of those enemies without being touched on the hardest difficulty setting, then make the achievement "kill 10 enemies on level 10 on the 'hard' difficulty setting without being hit."

9. Do not allow the player's save file to easily and accidentally reach a point where an achievement is no longer earnable. It is unbelievably frustrating to pass up a narrow achievement window, save your game, and then be unable to earn it without starting all over. It forces the player into a mentality of nervously using online guides to see when achievements are coming up [...] and then carefully earning each one before progressing naturally with the game. It completely sucks away the carefree enjoyment of just playing through a game's story, and there's really no reason for it. Let the player go back and earn missed achievements, if possible. If not possible, evaluate whether to have that achievement at all, or at least how it can be made difficult to miss.