There are 2 moves that raises a user's Special Attack by two stages
Geomancy raises the user's Special Attack, Speed and Special Defense by two stages. Takes two turns to execute. Signature move of Xerneas
Nasty Plot raises the user's Special Attack by two stages
However, If Arceus acquires Simple through the use of Simple Beam , Calm Mind will increase its Special Attack and Special Defense by two stages (courtesy of bulbasaur_ggd)
Arceus cannot learn Nasty Plot or Geomancy but by using the "Simple" Calm Mind trick he can gain a +2 in Special Attack.
Hope this helps!