So enough with the little stuff, time to find something more substantial. I saw an old heater/stove thing, I don't know what it is but it was big and metal.
I started trying to hit through like I did with the sheet metal, but this was much thicker so I started hammering the butt of the knife with a 2x4.
I finished cutting a hole in the side of the heater. There was another layer of metal under the outside layer that ended up chipping the point of the knife. You can see in this pic about 1/16" is missing from the point.
...and some more pics of the damage
At this point I felt I needed to chop something, and I saw a nice, heavy concrete block.