My understanding of FR pots is that they mitigate fire damage and they are done. So, in Onyxia I would wait to quaff until phase II, when normally two 3000 HP fireball would be lethal. The first fireball would only hurt a little, and the second would at least not be deadly. I very rarely ever get hit by more than 2-3 fireballs in phase II, and this is another way in which it helps to have more people. The little FR pots are easier to come by, and with bandages can mean the difference at times.
Resistance seems to be a some or nothing deal, so it improves your chance to take less damage. My experience is that resistance is calculated based on the level difference, so I don't think there is a good chance of resisting Ony's fire attacks. It might help more in resisting the lava vents.
Resistance seems to be a some or nothing deal, so it improves your chance to take less damage. My experience is that resistance is calculated based on the level difference, so I don't think there is a good chance of resisting Ony's fire attacks. It might help more in resisting the lava vents.