The point of this thread's suggestion (which I disagree with btw) is to make EV training easier (I would assume). It's terrible trying to spdef train on tentacools.
But on-topic: That's what your useless illuminate starmies are for!
This is an easy way to remember:
All types that eevee can evolve into (including grass/ice as in gen 4) are special. Dragon too.
Everything else is physical.