I'd be inclined to drop the lever though it should not matter. The tightness can only be the spindle in the barrel assy or the two pins at the bottom.
If it has not been stripped for a while I expect its gum or light rust on the spindle stopping it from coming apart. Once stripped and oiled you will find it pulls apart dead easy.
You could try some WD40 around the spindle to help it come apart before using the mallet.
Drop the lever, grip the barrel and tap either side of the front of the cylinder.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane