If you're only doing Northern Exposure, you only need one of these. I've categorized them by area so you can find the one in the area you're questing. Once you've done Northern Exposure, you'll unlock Frostbitten, which requires all of these kills. Whichever one you're doing, here's the list:
Howling Fjords:
Vigdis the War Maiden
Scholozar Basin:
Icecrown:
High Thane Jorfus
Hildana Deathstealer
Borean Tundra
Old Crystalbark (Coldarra)
Fumblub
Storm Peaks:
Dirkee
Vyragosa
Dragonblight:
Crazed indu'le Survivor
Tukemuth
Zul'drak:
Zul'drak Sentinel
Terror Spinner
