Instead of relying on regi, why don't you replace him for a wish-passer chansey? That way you get 2 good walls, and donphan gets a way to recover itself.
A good set would be this:
Chansey @ Leftovers
Natural Cure
Calm (+Sp Def, -Atk)
252 HP / 252 Sp Def / 4 Spe
~ Wish
~ Protect
~ Seismic...