Most will probably realise this but just incase anyone doesnt, these achvs became much easier to obtain with the account wide achievements
If you have a few lvl 70+alts you can park them in the areas where the mob you need spawns and log each of them in turn to do a quick cycle of the area (npc scan and overlay are very good add ons)
I needed about 8 mobs from each achv and got them in 2 days by doing this,also im sure when you kill one another spawns in another location so if you kill one on a toon in icecrown for example then you will have a better chance of getting the mob that has spawned where one of your other toons are camped.
Again just coomon sense but incase anyone hadnt thought of it?