How to level a section of raised concrete floor?
Grinding concrete is a very dirty job. If I had to do it I would use a 9 or 4 1/2 inch grinder with a concrete grinding wheel. At least an N95 dust mask, goggles, preferably like motorcycle riders use with the foam around the edges to keep dust out. I would put one or two shop vacuum cleaners hoses in the exhaust vent to create an air flow through the bathroom. You will need to mark where the toilet base hits and remove the toilet then grind away free hand. Leave the outside edges till last so you can put a straight edge across to measure progress. Leave the area where the toilet sits high, it wont be noticible when the tiles are in. Can you get the plumber to fix it, he screwed it up.  For the vacuum cleaner's you could remove the door and make a plastic door then you could just run the hoses under the plastic and let air be pulled in from the vent, try and have a slightly negative pressure to keep the dust in the bathroom.