One of the more time consuming set of achievements to get is the rare spawn killing achievement, if only because you need to compile information on every spawn, then head over to their spawn point and wait, then hope you grab the monster before someone passing through sees the Silver Dragon and gets the loot gleam in their eye.
We can't really help you with the last two, that's up to you. What we can do is help you with the first. For the Northrend achievement, Frostbitten, that we have a map submitted by tipster Mike, player of Paksenarrion on Bonechewer. It's a hand color coded map of Northrend that shows where each require rare spawns. It's pretty basic, and Mike admits he took most of spawn information from sites like Wowhead and Thottbot and has only managed to find about half of them himself, but it's a nice start.
Myself, I doubt I'll ever seriously pursue Frostbitten unless It's the absolute last achievement I have to work for or some such. Still, for those of you who seek the bragging rights that come with a massive luck-based time sink, maybe this map will help. And if nothing else, Northern Exposure only requires you kill one of the rare spawns, so knowing thier spawn points can at least help you keep an eye out while you quest and pick up that quick achievement.