This may seem pretty daunting, but its surprisingly easy. Playing through all of the quests for Frozen Wilds will get you 6 - 7 of these kills right off the bat. To get the rest, there is a Frostclaw site east of the Deep Din. You should see about 3 to 4 of them spawn there. I was able to take them out almost all at once by using the blast bomb from my Lodge Blast Sling (for those wondering, I have all three mod slots filled with the highest level damage coils I could find). After they went down, the trophy dinged.