Regarding this question, an Ice Embermage. The first question appears because the debuff is per projectile, so you can gain -100% resist quickly by hit enemy with icy blast in near range without maxing staff mastery. And It's cumulative right? So it stacks by itself? (I hope I get the information correctly in my mind)
The second question is like this.
At first I focus points on Icy Blast, Staff Mastery, Ice Brand, and Charge Mastery. Then I usually leaves the passives at lvl 5 then go to active skills. So as you can see when I got to The Temple Steppes, where the amount of enemies is increasing drastically IMO, I still have Icy Blast, Staff Mastery, Ice Brand, and Charge Mastery, with 1 point I spent on Frost Phase. So because of this, my damage is high but my mob controlling is kinda terrible. Consider I have tried put 1 point in Hail Storm and Frozen Fate, the mob controlling is good (from stun and freeze on kill) but the damage for single enemy is terrible.
TL;DR, killing speed or mob controlling?