Between tanking NMs and waiting for the next phase of the Whispering Tide, I’ve been working steadily to hunt down all the rare monsters in The Secret World. There aren’t nearly as many as in WoW—I think there are only around 40—but it’s still pretty time consuming since you have to wait around for them to spawn. Some are on short 15 minute timers, but with others you could be waiting 3 hours. Still, there’s a sweet jacket at the end of it, so I feel like it’s a side project worth the time!
I’ve been using the rare spawn maps at the TSW wiki and browsing info at Unfair.co and this forum thread. So far I’ve downed everything in New England and the first zone in Egypt. Making good progress! I haven’t had trouble finding anything yet, but I’ve definitely spent some time waiting around for them to pop.
Sometimes I remember to take screenshots, but others I’m too excited to see a spawn and then I forget! Here are a few of my trophies so far:
The Dunwich Horror. This guy took me some time, and I was so relieved to find him wandering the fence south of the airport in KM I celebrated by killing him.
Ainn the Harbinger of Death. I waited around for a while on this guy too (noticing a theme?). I actually like the design of this golem model a lot, looks crusty like it’s covered in barnacles or something. He spawns in the middle of a pack of worshipers, and I bonded with them while we were waiting by killing them all repeatedly.
The Lost One. This was a piece of cake! He was the last rare I had to hunt down in the Scorched Desert, and he was right there waiting the first time I went looking. So grateful for the convenience I gave him directions (TO DEATH).
It’s been a fun journey so far, and I think I’m about halfway done. I want to finish it before Tokyo because I’m assuming they’ll add more rare monsters, and then I might have to kill them for the achievement too!
MEGADIRGE 
It’s interesting that you mention a lot of waiting, for i did almost no waiting at all for this jacket. What i rather did was to mark all the places of those guys on all maps of the world. Whenever i passed by not too far of one of those places, i took a look there, and over the course of a month or two i had all but one in the besieged farmlands, where i then indeed did some “waiting” by visiting the same place again every 30 minutes and doing missions all around in the meanwhile.
Yeah, if I made it less of a priority and just checked stuff whenever I was nearby I would definitely have a lot less waiting! But it’s one of the main things I’m pursuing right now. Also, some spawns have a 3 hour timer, so if I go there and it’s not up then I have no idea how long I’ll have to wait.