WD40 may not be the best solvent for the job. A solvent that's made mainly to loosen screws, like Liquid Wrench or B'Laster, may work better, but they also might not work too well against the residues that build up on plumbing.
Soaking the head in a cleaner made to dissolve calcium, lime, and rust, like CLR, or vinegar, with this trick (saw it on Lifehacker)
![plastic bag trick to clean showerhead](https://i.stack.imgur.com/eLSNW.jpg)
may loosen things that the others don't. You fill a bag with cleaner and tape it onto the showerhead to let it soak.
You can grab the pipe with whatever wrench is handy, but the right wrench will grab more securely. A pipe wrench
![Rigid 31000 Pipe Wrench](https://i.stack.imgur.com/cFmVq.jpg)
will grab it for sure, but it will bite into the metal and leave a visible mark.
A strap wrench
![Klein S-6H Strap Wrench](https://i.stack.imgur.com/iJCZe.jpg)
will grab without marring, but the surface needs to be dry and clean - any wd40 residue would need to be cleaned off.
There are some cheap rubber strap wrenches available
![Harbor Freight Rubber Strap Wrenches](https://i.stack.imgur.com/em5gR.jpg)
Both types, strap wrenches and pipe wrenches only work in one direction - you'll figure out which way, just be aware of that.
The drain line is probably filled with hair and soap, chemical drain cleaners at best push the clog further down and make things worse in my opinion (now toilet backing up) with the exception of “mule kick” that will eat metal pipes and I am not sure if it is even available any longer. What you need is a real snake a small one to be run down the shower, and possibly a larger power one to be run down the toilet. Without cleaning the clog this will get worse this kind of snake you hold in one hand and spin pushing it down the drain a little at a time probably less than 2 or 3 chemical treatments. You should have the land lord take care of this incase any damaged pipe or other problems come up and you are not taking the $ hit for something that should be fixed by them.
Best Answer
You need more leverage. To do that use a pipe wrench with a cheater bar attached so the handle is longer. Slip a 1 inch (or comparable) diameter pipe length onto the handle. This will greatly increase the torque you exert. Counterclockwise direction is loose.
Apply force slowly and in increments. Most likely there's a sweated copper fitting behind the shower arm and it can break if over torqued.
A more passive way to remove it would be with a hacksaw. Make a diagonal cut on top of the squared shoulder of the shower arm. Cut until you see the 1/2 inch nipple that it's threaded to. Make the cut from its base to the end of the square shoulder. It will spin right off when completed.