If it's in a spot where I can burn it then I drill a bunch of 1" holes with a long auger bit. Top them up with diesel a few times and let it soak in good. On burn day I drill a few more holes from the outside edge down towards the center, the idea is to get a few air vents that connect to the holes from the top. Build a little fire on top and let it go. Mine go hours seeming like nothing is happening and then the top caves in. It'll often burn a couple feet of the roots if they're dry.
