Sentence:  Beating a difficult <WSD>instance</WSD> becomes second nature after running through it…a few times, with good leaders knowing exactly what to do and how to co-ordinate member actions. 
Meaning of the word marked with <WSD> tags:  A dungeon or other area that is duplicated for each player, or each party of players, that enters it, so that each player or party has a private copy of the area, isolated from other players.