I have to replace a cartridge in a delta single handle shower. It is a one piece lever. It has a set screw on the inside bend of the handle. I have removed the set screw and the hard water corrosion will not break allowing the handle to pull off. I have tried boiling hot vinegar, along with a gentle tap on the handle to see if the corrosion will break free. I am wondering if I should use a dremel and place a long cut parallel with handle to then insert a flat blade to split it off the stem.
Does anyone have a better suggestion?:):)

Take large screwdriver with plastic handle. Keep tapping the shower faucet handle with screwdriver's handle: few taps on left, few on right, few on top, few on left. Keep tapping it while you pull on the handle. After while you will feel wiggle. Keep tapping and wiggling. It will come out...

I would spray some WD-40 through hole where the set screw goes leave it for a few minutes to soak and try it wiggling, hope this will work, good luck.

John