Wall Shadow has to be in your Deck, not your hand. This makes it useful with the Labyrinth Builder skill to ensure you can summon it.
If they release Mangum Shield (http://duellinks.gamea.co/c/sn7ld1lv) this card will be intense. Already nifty with Labyrinth of Nightmares and Shield and Sword. I used to run this card (and Millennium Shield) IRL with Ring of Magnetism and use low attack monsters that can attack LP directly for a frustrating combo.