If this connection is that difficult to separate the splines on either the shank or the faucet or both are damaged. Not crazy about the shank with the straight nipple of the backside. The beer line tend to sag and possibly kink.
You may have to put the shank in a vice (wrap the threads with cloth to protect) and attempt to dislodge the faucet with a rubber mallet. If successful, inspect the splines for damage. If after cleaning the two pieces still do not seat correctly, you might want to order a new
shank and / or faucet to replace.