Well contrary to what you're doing, training in a Pokemon's weak spots is generally a bad idea. What you want to do is play to Sandslash's strengths.
Since Sandslash has naturally low Special Defense and Speed, even if you EV train it, it isn't going to be that great. What I would say, is to EV train in Attack and HP. Sandslash has a good attack stat, as well as acceptable HP, and by EV training Attack it becomes a better offensive Pokemon, and training it HP gives it extra bulk, allowing it to take more hits - both physically and specially.