Diablo – When the effect of invigorating gemstone procs, does it do so before or after taking damage

Elite monsters are any monster which have special attributes not normally found on their base type. Champions, Rares, and also Superunique monsters are all considered "elite".

Source: http://www.diablowiki.net/Elite

I finally noticed a pattern with the corrupted angels that explains the damage reduction I was seeing.

When a corrupted angel first takes damage, it gains a shield that reduces all damage of that type by ~66%

In the screenshot below, you can see one angel has a reddish shield. The shield was not there until I attacked it with a fire-based attack. The other is a little harder to see but it has a shield tinged with blue. I hit that angel with a lightning attack.

My additional fire attacks on the red-shielded angel did about one third of normal damage, but the lightning hit for a normal amount. However, the blue-shielded angel took roughly one third of the lightning damage, but took the full force from the fire attack.

So if you are progressing through Act 4 and having a tough time on these creatures (especially champions), the answer is to hit them once with a secondary attack that deals a different element than your primary damage dealing attack. They'll shield against it, and your normal attacks will deal full damage.