Shedinja is... dead?! LOL?
Since you can't alter it to increase it in any way, I thought of ways to de-crease it. :P
To answer the real question, the HP IV cannot be increased & decreased in-game, only on online battle sims like PO or Showdown. The IVs are set in stone one the Poke is hatched from it's egg, caught in the wild, or given to the player as a gift.
It does not matter if Shedinja's HP IV is 31 or 1, it will always have 1 HP.
It's impossible to do that, Shedinja's HP will always be 1, no more, no less
You can't raise it's HP with anything or use a Focus Sash or anything else that prevents fainting
Source: Personal Experience and http://bulbapedia.bulbagarden.net/wiki/Shedinja_(Pokémon)#Base_stats