4" diamond wheel on the grinder with no guard as Pinkboy said.
But I did it in-situ.
I waterproofed the slab and stuffed the floor drain with paper. Then I took location measurements of the floor waste and screeded over it as I did the floor. I then laid the tiles and grouted them. Then I marked on the floor the position of the concealed floor waste and got to work with the grinder.
It could have gone horribly wrong.