You can use various items, such as Epsom salts, a high-nitrogen commercial stump rotting substance or cow manure for a natural high-nitrogen choice. To add such a substance, you’ll need drill lots of holes across the stump’s surface, then add the rotting agent. Put a tarp over the top and cover with mulch to help keep in lots of moisture (also be sure to add lots of water). Then, all you can do is give it time, perhaps adding more rotting agent a few months later to keep the process going, as well as ensuring plenty of water is getting to it. This will be a slow process, but it’s a natural one that lets you avoid having to do a lot of heavy work. You might speed it up by uncovering the stump occasionally and re-adding the rotting agent and lots of water, then covering it again.