To my understanding there's no way you can be immuned to damage.
Every resistance is capped at 85.
A feral ghoul reaver's melee is 50dmg in normal mode.
DMG - DR 85% = 7.5 post DR dmg
7.5 post DR dmg - 85DT = -77.5
So final damage taken is using the post DR value X 20% = 1.5
Deathclaw is always fun, from my experience in vanilla FNV it seems to ignore DT but not DR. Chems help to last 1 or 2 hits more. It seems to have its own special "FMinDamMultiplier" value(default 20% applied to post DR, post DT), the thing is its base melee damage is so high that makes all these irrelevant when it gets close. The only way up close with a deathclaw would be using melee special attack that knocks down.