First, you should google "game design document". That's the attacks, battle system, etc. The actual script for a videogame is much like the script for a comic book or animated movie, it's mainly dialogue and visual description of characters and setting. If the story is non-linear you might want to use a flowchart instead of a standard linear script. Game scripts do have storyboards if they call for cutscenes (movies that play within the game). Many games use scripted scenes instead (this uses the regular character models or sprites of the game for plot scenes instead of making a movie with original animation.
But, you cannot pitch and sell a video game script in the same way you could a movie script. Big-name video game companies pretty much don't buy scripts, they write them by committee much like Disney does.
