General

@dzakien:
It might have been changed with the recent collection changes to be given when you collect them all? Or 30?
idk. I haven't moved to Mu Lung yet.

That, or an exploit.

@littletlk: No. Listen. There are only 38 monsters in Mu Lung, Collection-wise. How in the world is the medal achievable then?

@dzakien:

Obtaining 50 monsters in any area gives you a special medal.
(There's another at 100 monsters in a single area)
Some of these medals are obviously unobtainable until they add more monsters.

@littletlk: TLK. tf is this http://i.imgur.com/xYJxyTQ.png
I'm avid collector myself and have my head in wraps.

I have a lot of experience with Monster Collection.

Higher level = lower chance to add. By around level 150, it's insanely low rate. You'll be at it for double digit hours in the average case.
Bosses (and minibosses) have a very high add rate to compensate their low spawn rate/ability to only fight once a day or once a week.
Elites do not have a 100% add rate. So even if you spawn one, don't expect an instant-add.

Contrary to what some are saying, there is NOT a set number you need to kill. It is purely random per mob at a fixed %chance.
I'd estimate that level level mobs (1-30) have something like a 1-2% add chance and mobs in the 80-100 level range have something like a 0.01%-0.05% chance to add. Bosses seem to have around a 1-10% add chance depending on the boss.

I spent 10+ hours for the Thunder mob in future hene. Make sure it's not an elite version you need btw.

@solzoriana: according to an insoya post on KMS I read up a while ago, the higher level the mob, the more kills that is needed to collect. Level 120+ and Level 160+ isn't any different though, which is why the low level hene mobs were super easy. Also, it seems that there is a bit of RNG tied to the # needed to kill. Each account supposedly rolls for a number = the kills needed. So sometimes high leveled mobs takes very few still, because you rolled a small number, but they have a higher cap, which is why sometimes it takes hours. This was all information I got from an insoya thread a few months ago before I first started doing monster collection.

I've also found that higher leveled lobsters are harder to add to the monster collection. Though I wasn't actively trying to add the lvl 230 upper right stem mobs to my collection, I was finally able to add one after about 5 hours of grinding (with totem, too!).

@javirocks: So theres an actual amount of monsters that needs to be killed for it to be collected? Or is it just purely random?

While the collection rate can sometimes be random, the higher the level of the monster, the more monsters that need to be deeated in order to obtain it in the collection

Probably takes a while. Monster Collection stuff tends to be weirdly random anyways. I would just consider whether it is worth the effort to do it.