Every bull I had ever taken was pretty easy, and my Havalon just slices right through it from the tail to the back of the head. But two years ago my daughter's bull had the thickest neck skin I've ever seen. It was like heavy thick cartilage, and the hair was all matted up like a buffalo. In fact, when we dropped it off at the taxidermist, he brought out a cape he had there and commented how similar that one was to my daughter's. And he said......."this one is from a bison". They looked like brothers.
I actually broke 4 Havalon blades trying to cut from just the shoulder to the base of the skull. Then I pulled out my 4" fixed blade S30V knife that will slice paper, and it would only cut about an inch at a time with both hands pulling up on the knife. IMO not much else you can do when you get one of "those bulls". And ya, my cut zig-zagged as well. My taxi did a great job on it though.