Replacing Delta single level shower lever

hi
I am in the process of replacing my 1400 Delta series single level shower cartridge (RP19804) . I have to remove the bonnet nut (RP22734) in order to remove the old cartridge and according to Delta it simply needs to be unscrewed. However I cannot unscrew it. I tried to use a wrench but after applying enormous amount of pressure decided to stop as I did not want to break a pipe. Is there an easier way to do this.

Please advice.

Delta

Yes there is. Please no pressure.
Tap all around the round cap with the wooden hammer handle.
I use a 14" pair of Channelocks with the round jaws. With them when I turn I don't squeeze the cap, I turn the cap. It's easy to squeeze too much.
Put some grease on the threads inside that cap when you put it back together.
After you put the cap back on, adjust the tension on the ball with the plastic bushing.

thanks, for the advice. do u also recommend that I soak the existing cartridge along with the bonnet nut in vinegar and warm water. There is definitely mineral deposits in the cartridge etc.
The shower head has a drip, thus i wanted to replace the O rings in the cartridge. The drip starts when my sprinklers come on and this happens only in the shower faucet and no where else in the house.
This lever doesn't have a ball.

Appreciate your reply