I do know that nuking the mastermind while invurnable does NOT get you to the Apostle, but it does get you to JC.
I do not know if one needs to wear the berserker armor/Dragonslayer, but according to word around the forums, I do believe you need to wear/equip both beforehand. And getting such a rare drop sequence (DS and Berserker Armor) is hard enough to achieve.
As for the hitpoints, I have heard differently. I have heard/read that the Apostle does do massive damage to you, but his hitpoints aren't much more than a HMP/UV Cyberdemon.
Best of luck, monster hunter!