How to Reach World 8-7 in New Super Mario Bros. Wii

World 8-7 does not exist in New Super Mario Bros. Wii.

The game's final level is World 8-4 (Bowser's Castle), where you defeat Bowser and rescue Princess Peach.

You might be thinking of a different game (e.g., Super Mario Bros. 3 has World 8-4 as final, Super Mario World has Star World/Special Zone). Double-check the title and level number.