It's difficult, because of the rubber and plastic seals in close proximity, but I generally use heat on female threads, and try to cool the male component with application of cool water rag. This takes advantage of expansion of material with heat to increase thread clearance.
Also, I would introduce a good penetrating oil, (I use Kroil), along with some light tapping to encourage the lubricant to penetrate. WD40 is NOT penetrating oil.
If I wasn't confident about grabbing the perimeter of the cap with a pair of Channellock pliers, ( about a 10" curved jaw, or V jaw seems about right)
https://channellock.com/pliers/tongue-groove/ ,
Then I could drill the cap at two spots and use an adjustable pin spanner.
https://www.amazon.com/Trades-Adjust...df_B00V539YTE/