My consulting saw jockeys are unavailable to check with, but I think I would begin with my chain saw and plunge cut a grid of vertical cuts leaving multiple columns of wood to be broken/cut out with suitable tools.
If there is enough ground clearance, a horizontal plunge cut or two could do the trick: think pick-and-pluck foam gun case liners.
Starting a plunge cut is about as risky a cut as you can do with a chain saw, so read up, consult a pro if one is available, watch the instructional videos on YouTube, and take ALL safety precautions.