Use the old diaphragm as a pattern and drill some holes in a piece of metal, 1/4 aluminum or steel. I would use at least 4 screws. Screw the assembly to the metal with screws that will go in deep and tighten down. Add a washer or 2 if you need more depth to tighten the screws. You can now clamp it in a vise or bore a few more holes and bolt it to a bench. You may get away with a big adjustable wrench but whatever you use it needs to hold the nozzle well. It's brass so a loose wrench will likely displace metal and you run the chance of damaging the nozzle. Most wrenches do not have a deep enough throat for this job. Don't use a wrench unless faces of the hex are at a minimum 100% covered by the wrench.
If you don't mind a shameless plug, I make a pull bar specifically for this operation. It works on all 1in nozzles and yoke nuts. It's in my 2019 catalog.
With that said, you may want to NOT remove it. The only thing in there is a crush washer, if it's not leaking, there is really no need to separate the 2 parts. If you separate the pieces, the washer may not reseal and I have not heard of anyone finding replacements. Not a lot to gain and a lot to lose.