Arch pain can be caused by many different things:
1. Boot is too thin somewhere, which pushes the arch(es) of your foot higher than it is are comfortable.
Based on your description, this seems the most likely explanation. But read the other possible causes, and see if you can rule them out.
As I said before, if the thin part is at the very bottom of the boot, it can not be stretched, because the outsole cannot stretch. However, if it is a few mm higher, it can be stretched somewhat - maybe by a couple mm. If a little higher, it can be stretched a little bit more.
And as I said, I think your boots are too stiff to do a stretch fit just by wearing them - though you may want to give them a week or two, if you haven't already. You need heat, pressure from a boot punch, and possibly moisture or alcohol - though neither fluid is good for the leather.
You can do this yourself too, using a somewhat less expensive tool: a ball and ring pliers. Also called a hoke and ball pliers, or a cast iron bunion stretcher. E.g.
here. See my web page for directions, though it may be easiest to let your most expert skate tech try to do it, though if the stretch is too large, it will keep unstretching, so you may want the tool to restretch it every month or so.
Another fix: if you remove the insole, that will cause your foot to sit a few mm (at most) lower in the boot, which is normally slightly wider. Beware - if there are nails protruding from the bottom of the boot, or anything else rough or sharp, you need something to replace the insole that is thick enough to smooth things out.
You can combine all of these things.
If it is #1, and it is impossible to stretch the boot enough, even if you remove the insole, unfortunately the cure involves buying another pair of boots.
Given that skate boots cost a lot of money, this causes a different type of pain.
2. The arch of the boot is significantly higher than the arch of your foot. Same result. Can be fixed by altering the shape of the insole, or making a new one, with a lower arch. Or add tape or adhesive foam under the sides of the insole, to bring them up.
3. The arch of the boot is significantly lower than the arch of your foot. This forces your the arch of your foot to flatten more than is comfortable. Same fix, but make the insole with a higher arch - or just add tape or adhesive foam under the middle of the insole.
4. The sideways (left to right) tilt of the bottom of the boot, including the insole, does not match the sideways tilt of the bottom of your foot. The way I see it, the primary effect is to force your muscles to compensate, which eventually (it takes a while) starts to cause muscle soreness. Once again, you can make a new insole, or modify the existing one.
5. The fit isn't tight enough under your arch, and you are slipping under the arch, and getting a blister. Same fix as #3.
6. Maybe there are other things that I haven't thought of.
If you can't figure out which problem affects you, experiment. Doing the wrong thing will probably make the pain worse, doing the right thing will make it go away.
Good luck!