There's no way you cannot get a tyre off if is the correct size.
Make sure the inner tube is well deflated, push the tyre in the rim well, you have to create a sag. Put a tyre lever in, make sure the inner tube is not in the way, hook this tyre lever on a spoke. Then add another tyre lever, about 2" away, the trick is to go in small steps: 2" per time