Cutting into big cactus plants has always been difficult. It turns out that I found a tool, called an oscillating multi-tool at Harbor Freight for about $20 on sale that does a wonderful job of slicing through cacti with minimal crushing damage to the plant, especially so in light of the woody structures that form around the vascular bundle of any large-ish plant. These structures make it difficult to cut through the plant. I liked the tool so much that I upgraded to a battery powered variable speed model, costing around $50. It's business end is illustrated. There are various attachments for the tool for use on other jobs. I'm pretty sure that I'm the first to use the tool for cactus surgery. Illustrated is essentially a saw attachment which I've found very good at doing cactus surgery. It amounts to being an electric hacksaw for wood. You probably don't want to spring for this tool because you don't have more than one or two plants that need attention. You can do fairly well if instead you use a hacksaw to cut big cacti. Buy a new blade, and don't push the saw very hard. Alternately you could use a very sharp knife to cut to the woody center of a plant, then use some sort of cutter to get through the woody bundle, finishing off with the sharp knife to cut through the vascular bundle.