If you'd take the time to explain this in the first post there wouldn't have been so much confusion
Due to shield block having a cooldown adding 15% more connect chance with your styles won't hinder that boss from eating you sometimes, resulting in a most certain death - especially if you can only survive two unmitigated hits right now and sit at 5% afterwards. Lowering your healthpool will only increase the risk of dying in that situation even without raiddamage going on. Halion had parry haste enabled, i'm fairly certain. But if this is still the case.. just check your logs, nobody bothered going back and testing this after all those times/patches.
But to keep it easy, just bring two set of gems/trinkets and see how it feels? I really don't know how much a 36 stam trinket on a level 70 adds for his healthpool ~
Ps: what happens at 20%? Does the boss enrage, thus hitting harder/faster? I really don't remember, but if it's nothing like that you should just keep trying - or is there more like healers running oom etc?