It depends on the diameter of the stump and how tall it is, My OH suggests drilling large holes with a corer very close together and then chiselling out and then repeat until you have the required depth. You will need to think about drainage :maybe drilling holes through the sides at the lower depth. Then think about the rotting of the stump unless you treat the hollowed out area with bitumastic paint. Good Luck

Why not make it quite level and put a suitable sized pot on top? Much easier.

Call an arborist and they will grind it out with a giant rotary wood chipper. They will even go below grade and remove all traces of the tree stump in no time. You will be amazed by the amount of space that old tree stump took up, which is now freed up.