OK, so as we all know using the skill Shattering Scream followed by Blast Shot you get the combo "Cruel Shot". Now I havent tested this theory at all ,but i think that you can get this combo a different way as well. Lets look at the discription of the skills.

Shattering Scream (at rank 5 )- Reduce DMG -30, Armor -25

Blast Shot(at rank 5)94-118 damage,6m area blast

Ok. So what i think is that you can get the combo "cruel shot" by using Blast Shot after any kind of armor DEbuffing skill. For example i was playing and a bear was using a spear(which means that his spear was debuffing opponents armor because of spear proc) and i followed with a Blast Shot and the combo came up. Ive also noticed the combo coming up at other times too , in which i didn't see the Shattering Scream being used. As i said earlier i havent tested this,its merely a theory. So idk what you guys think about it. Just thought id put it out there.