User blog comment:Gracchus Mccarthe/Invincibilty/@comment-6016076-20141126184406

%DR from armor = sqrt (total armor in that damage type)

Combining Tough Body %DR with armor %DR =  1 - (1 - %DR Tough Body) * (1 - %DR Armor)

For true invincibility, Tough Body doesn't matter. The damage reduction percentages between armor and Tough Body multiply by each other, and you can't reach 100% by multiplying two percentages less than 100%. In other  words, you always need 10,000 armor to get 100% Damage Reduction, regardless of how much True Body is skilled. With 175% bonus armor from Body Armour Expert, this comes out to 3637 armor.

That said, you can get practically very close to 100% in conjunction with Tough Body. For example, at 7945 total armor (2889 w/out b armor expert), you will have 95% effective damage reduction. A RED Heavy Trooper Vest with 10 Foritifed is already 2290 physical defense on its own. If you have a medic pal, you can also get as much as 1250 bonus Toxic and Heat from his Protective Aura. So "feeling invincible" should be well within reach if you're maxing Body Armour Expert, unless there's some cap on %DR that I'm not aware of.

As always, dark minions ignore armor, making it impossible to be invincible against them. Although you can combine 25 Tough Body with Die Another Day to achieve 95.4% DR while the buff lasts.