If in doubt... If you have access to a bench vice
Then the best and safest method is to remove the caliper body itself.
1. You have to disconnect the flexi brake hose to the caliper itself but since you are bleeding the system anyway, it's no great hardship.
2. Just remember to clamp the flexi hose to stop air getting all the way up to the ABS valve.
3. Once its unbolted and loose from the car give the bleed nipple a good squirt with "plusgas".
You don't need it to soak in for long... Even giving the end a gentle tap with a hammer can help free off a siezed thread, especially if you are changing the nipple anyway.
4. Now place the caliper bleed nipple in the vice jaws and tighten up as much as possible, even using a longer tube as leverage on the vice handle.
5. Get a good firm grip of the caliper body and twist the nipple out gently with firm but gentle pressure until it starts to release.
6. Switch to a spanner to take out the nipple once released to preserve the thread from damage
7. Replace the nipple with a new one but make sure that you give the "threads only" a light smear of copper grease making sure that you keep it away from the internal bleed hole.
8. Don't over tighten it and fit the caliper back on the car in reverse order.
9. Bleed the brakes as normal and double check for leaks around the brake hose and nipple, they don't need much to nip up
This also works on rounded off and chewed up bleed nipples too