What tool should I use to cut through a wall to get to my attic?
Yes, a drywall saw is the tool to use. But for a hole big enough to crawl through, I would not bother with 45 degree cuts. It's more work when making long cuts and is not secure enough to support large patch panels. Cut out a panel all the way to adjoining studs. Then cut out an additional 3/4" strip each side so the replacement panel can bear directly on 1/2 of each stud. You can use a utility knife and chisel to remove the strip. You will hit screws or nails while cutting the strip, remove them as they are found. When installing the new panel, use a new fastener on each side of the joint where ever you found a fastener.

The plain butt joint at top and bottom is not a problem because your patch panel is spanning between studs. You may want to consider installing a removable plywood panel and casing the opening rather than patching the drywall.